Guitar amplifiers will often have multiple channels or effects that can be switched on and off with the use of a footswitch. A 1/4" jack is common for the footswitch input on the amp that is either TS (single) or TRS (double). A TRS input can accept 2 amp channels or effects. Using a TRS footswitch with dual switches will control both channels / effects. Often the left switch goes from clean to drive, and the right switch turns reverb/chorus on and off. A TRS footswitch can be used for a single switch amp, just the left switch will work.