Mirza,

A compressor is something that you really can't or should tell that its on, it going to smooth out the peaks in the volume of a singer or instrument. It like having an automatic sound guy riding the fader so you can have a more controlled consistent volume level.

If I'm correct the behringer also has a peak limiter which would allow you to set a certain level point which the behringer will stop peaks from going over.

Also try reading the white papers at dbx for basic compressor theorys and set ups

Hope this helps