Gate / Noise Cancellation
A gate filters silences the output if the input signal is quieter than a configured level.
Noise Cancellation is a common application of a gate. By setting the cutoff level to a level that is louder than the buzz and noise when no note is being played, yet quieter than the quietest note that you will play, it suppresses the buzz and noise when you are not playing.
A poor quality device, or one with the cutoff set too high, can cause notes that you wanted to be heard, to be suppressed.
More sophisticated gates will allow you to adjust the response curve -- how the volume ramps up around the cutoff point -- and its response time -- how quickly it turns the volume up when it detects a loud enough signal.
It can sometimes be useful to trigger the gate using a different signal from the one it is gating. For example you might have a guitar line pulsate in time to a drum.
