A sequencer used for music is a device that sequences musical events for predictable playback. For early electronic instruments these sequencers often take the form of a collection of switches that are cued in series and which trigger some other device like an VCA. The switches may also be used to control a variable voltage source which controls the pitch of an oscillator. The sequencer most commonly has a number of switches that is a multiple of 2 like 4, 8, 12, 16, and has a looping function so that the series of switching may restart as soon as the last switch in series is triggered.
Below is a general schematic for the sequencer found in this youtube video: