what does the boss CS-3 compression sustainer pedal do?
what will it do to my guitar tone/sound?
Increases sustain, compresses loud signals and boosting low signals, making the sound...even.
Gear: ESP Horizon
so say if i use it with some rhcp songs...
i can use it in a clean solo (like californication) and it will make my guitar sound different?
I like compressors, especially when playing clean. It wont change your tone, but it increases the time the tone rings, and when you pluck the string carefully/smoothly/whatever it boosts the signal, and when you strike the strings hard it compresses it.

Watch and hear sound samples here.
Gear: ESP Horizon
prolly a waste of money for you until you develop your ear for tone
Get off this damn forum and play your damn guitar.
Basically the same as a piano sustain pedal, press it, it holds out the note for you, its for lazy people mostly :P

In conjunction with delay and expression pedal it can have a massive effect makig it sound spacey and cool

Uncompressed sound - ZzzzZzzZzzzZZZzzzZZZ
Compressed sound - zzzzzzzzzzzzzzzzzzzzz

Thats what a compressor does; levels out the volume. The sustainer is something thats usually built into the same device which sustains the note. There are tons of topics on this already so search. I'm sure there are better answers too.