this here is a great course about compression and dynamics:
https://www.creativelive.com/courses/compression-dynamics-master-class-graham-cochrane
first question for me during mixing, do i want compress this track and why?
are the dynamics out of the place and i want to even them out? thats good reason for me to apply some gently compression..
are the dynamics ok? then i most likely dont want to compress the signal, but i do want more excitment from the signal? if yes.. then add some distortion/saturation instead of compression
you can also do both, first add a compressor and then some distortion
understanding compression can be difficult first, but imagine this:
a compressor is a tiny little man in the box (plugin) that turns the volume fader down once a signal gets too loud (threshold)
the ratio is how much he is going to turn it down
the attack is how fast he is going to turn it down
and the release is how long does he take to turn the volume fader up again
i think a good visualisation of compression is when you are using plugins with a compression visualation gui, like pro tools stock compressor or izotope alloy for example, you can dial in the compression and you see how the little man in the box turns the volume up and down, see the attached screenshot just as an example, the yellow line at the top next to the waveform is how much this little man in the box turned the volume down on each time the threshold has been exceeded
the goal is not a total flat signal, the goal is just to compress it gently together, so that you dont have too high spikes, you are reducing them and you can bring up the volume of the whole track (make up gain) and because of that you will be able to hear the quieter parts better