What is a mono amp?

A mono amp is an amplifier that puts out only one channel of sound, as opposed to a stereo amp that puts out two channels of sound. A mono amp is generally used for extremely low frequencies (ie bass) that are channeled into a single subwoofer. It doesn't matter that both stereo channels are combined for these low bass frequencies because you couldn't tell which channel the sounds are coming from anyway (because you feel the sound pressure more than hear the sound).