geeee U have loads of old post about this...

Basically a compressor is a tool that will tame yer loudest bits in a sound, thus making it more "quiet and stable"... so basically u have a signal that as loads more consistency of levels, by consequence u have a louder average level (the way we hear is by "rms", meaning average, so basically u don't hear variations in level up into + - 3db).

Take the example of a band with a singer that has a vary unsteady vocal level... sometimes the voice will overpower the other instruments, others will be overpowered (ie not heard over the mess of other sounds)... this is a perfect example of why u need a compressor!

Image a compressor like some1 grabbing the level fader and turning it down everytime u speak 2 loud then bringing the level back in with yer voice is back 2 it's normal level...

of course u can compensate and control how much it brings the levels down, faster or slower, etc...

after the comp stage u get a make up gain, which means that u can make the sound louder (because a compressor does turn down yer volume but as u don't have 2 worry so much about levels going everywhere, u can the crack it up again)

Z