For Quentin and anyone else who finds it helpful.

You can probably find this information online pretty easily but I would like to try my best at a very simple explanation and relating it directly to the application of singing.

Basic function of a compressor - to reduce dynamic changes. It can also serve an effect function, to create a more squashed, saturated, in your face all the time sound. Put it toward the beginning of your effect chain. There are even more functions but they don't really get used on vocals.

Work through the parameters in this order. Not every compressor has them all, nothing to worry, just move on as normal.

Input gain/pre-gain - the volume of what's coming into the compressor to process. Set this and forget it, it's a hindering variable that affects the threshold.

Gain meter - this is not a setting it's just a signal of how much you've compressed. You can reference it while setting the threshold and ratio to take a look at what you're actually catching and compression, when in the audio, and how much.

Threshold - At what volume the compressor kicks in. Everything ABOVE (louder than) it is compressed.

Higher threshold - the compressor will only compress the very loudest moments - softer passages will be left untouched.
Lower threshold - the compressor will still compress the quieter moments as well as the louder moments.

Hard knee/soft knee - How gradual that threshold is. Hard knee means it starts compressing exactly at that point with no ramp up to it. Soft knee means it starts compressing a little bit before and a little more after - it just hides the threshold more.

Ratio - How many times the compressed dynamic range is being reduced.

For instance a 2:1 ratio would reduce a 6 db dynamic change into a 3 db change. A 3:1 ratio would reduce a 6 db dynamic change to a 2 db change. A 4:1 ratio would reduce it to a 1.5 db change. And so on. (this is theoretical btw and not important to think about the numbers) And it works downward - meaning the highest volume is reduced, not the lowest volume brought up, assuming you are not using auto make up gain which I'll explain later.

Lower ratio - a bit less dynamic range than the original
Higher ratio - a lot less dynamic range than the original

Remember this (as well as attack and release time) only applies to all sounds coming in above the threshold

(side note: this Ratio is called Headroom on the garageband AU compressor and shows up in db - weird, but it's the same thing. the visual of the plugin should make it obvious how it works.)

Attack Time - How quickly or slowly the compressor activates after processing a transient (quick pronounced spike in volume often starting a note or word)

Long attack time - the attacks will sound like they're accentuated in a very prominent thick way (this is actually an illusion but that's not important to know)
Short attack time - the attacks will sound like they're smoothly compressed along with everything else.

Release Time - How quickly or slowly the compressor makes dynamic changes (for the non-attack portions)

Long release time - dynamic changes will be made very gradually with the goal of diminishing the SOUND of compression
Short release time - dynamic changes are caught very early and accurately with the goal of flattening dynamics more completely

Output gain/post-gain/make-up gain/master gain - Basically identical to turning up the track. There are exceptions to this but they only come into play with more advanced mixing.

Auto make-up gain - I don't like it. Creates a weird mystery variable that I think theoretically would change as soon as you change the compressor. It's intended to solve the problem of compressors working only downwards and even it out so the dynamic goes both ways (loud parts made softer just as much as soft parts made louder), but IME, it doesn't work very well because it's done automatically and not by a human ear. Better to adjust the make up gain or track volume using your ear.

Auto-release - Not bad. I still don't understand how it works scientifically yet, but if you feel like the release time parameter is too confusing for you to think about, try it out, you probably won't be disappointed.

My personal recommendations for how to use these settings for vocals, notice I often don't say numbers because much of it DEPENDS:

Input Gain - whatever you need to get the volume right. Usually you can just leave it at 0 but if you recorded the track pretty low it's a good idea to bring the volume up to a reasonable level earlier than later and this parameter is one way to do it.

Threshold - I like to crank up the ratio and set the knee hard so I can clearly hear and see where the threshold is happening, and then set it low enough to catch even the softest phrases just a tiny bit (look at the gain meter). That way the dynamic compression is affecting the whole song at once (which is often what you need to get a vocal to sit in a mix easily and hear all the words clearly) not just taming the loud moments. But no lower than necessary, or else it will pick up too much room sound. Understand there is a time and a place for a medium of higher threshold instead, but think of a low threshold as a go-to for vocals.

The threshold should not be preset, you need to set it manually because it will ALWAYS vary depending on how loud you recorded the track and how loud you sang it etc.

Hard knee/soft knee - for vocals, probably the softer the better, but hard knee is standard and fine too.

Ratio - Depends on the compressor, the song, the style, etc. but there is a general ballpark range for vocals but remember this is specific to the compressor I use. I will usually use a ratio anywhere from 1.75:1 to maybe 3.5:1, 2.5 is kind of my go-to. There are times where I will crank it up WAY more though. If it's too low the volume will be out of your control and be too loud in some spots too quiet in other. If it's too high the vocal track will lose it's human feel and sound very bland. I tend to look at it with two mentalities - as high as I can get it without that bland sound AND as low as I can get it without the vocal failing to sit well in the mix throughout the song. It's a balancing act of those two ideas.

Attack time - Always as short as possible. Unless I wanted a prominent popping sound of the consonants, which I have yet to ever go for I think ahaha.

Release time - I like it short for vocals because I feel the dynamic changes come through in tone and texture anyways, it's nice and in your face, you're more likely to hear every word, it helps the compressor do its job accurately, it maintains the sound of compressor which I find desirable, and it prevents awkward automatic slow volume changes (for instance, a held note getting louder over time from a slow release). So for me, short means under 100ms. Be careful going shorter than 50ms, it can distort, but honestly, I do it all the time and don't hear anything wrong

Output gain - use your ear, it's your new vocal level.

Anytime you adjust the compressor you may have to readjust the output gain or track volume but it's worth doing that all hands on instead of relying on an "auto" setting.

Hope that helps and is concise enough. Any suggestions to explain anything clearer just let me know and I'll try to edit the post.

Cheers.