An amplefier makes some sort of singal stronger.
Your probobly thinking about an audio amplefier.This is the most comon amplefyer used,evry radio has one,You have an audio amp before you (the PC speakers) evryting containg a speaker needs an amplefier to drive it.
There are also other typed of amps.Like radio amps to bost comecial radio stations,and in radio recivers to make it more sensetive.They are also used to bost singals on long wires.Like there are amps evry so often for cable TV.
and amp is evryting that makes an signal stronger.