The looper is a web synth [module] that plays compositions created by the [midi-editor] in a loop with lots of options for configuring the lengths of the loops, switching between different compositions, and routing output MIDI events to other modules such as the [synth-designer]. It works in a similar way to the [sequencer] and in fact may one day be merged with it.
At the time of writing this, the looper is still under construction and some features may be incomplete or not work. I'm also actively working it at the time of writing this, though, so who knows.