So a compressor is actually a goblin in a box.
He has a little pair of headphones and listens to the incoming audio. He always keeps a hand on the volume control for the level of the audio going out.
His whole job is to turn the volume knob if the audio gets too loud.
The threshold is the level you tell the goblin: “If it gets above this loud, you turn it down, okay?” “OkAy”
The ratio is how much he turns it down. Low ratios (2:1) means he only turns down the volume a little, high ratios means he turns it down a lot and squashes the sound.
Attack is how fast he turns down the knob. Fast settings means he’s really on it, he turns it down right away. That means transients won’t get through. That may or may not be what you want. Longer attack times means he takes his time turning down the volume, which can help preserve transients and sound gentler.
Release is how long it takes him to turn the volume back up. Really short times could sound unnatural, really long times could mean you’re already on the next incoming transient while he’s still turning it back up.
Sidechain compression is when you plug his little goblin headphones into a completely separate audio source while his volume knob affects the signal you want compressed. So he’s listening and reacting to the kick drum, but he’s affecting the overall mix. (If that’s how it’s routed.)