'Feedback' is the word used to describe what happens when the sound waves from the speakers cause vibrations in the strings and/or the pickups, so that the original sound gets 'fed back' into the amplifier. With a little practice you can learn to control the amount of feedback you get by adjusting your amplifier settings and varying such physical parameters as how close you are to the amp, which way you are facing, palm-muting the strings, and like that.