Guitar Effects: Compression
Compressors for guitars come in pedal / stompbox and rack units, and are used to even the sound of the guitar's incoming signal.
This is done by adjusting the volume so the level of the incoming signal is "squeezed" or "squashed" to a more uniform volume level letting notes or chords sustain better and longer.
Compressors give other effects different characteristics when used together. Distortion or Overdrive and compressors are commonly used together.