Looper
An effect that records a phrase and repeats it in a loop, so a lone musician can stack layer upon layer of themselves.
One press of the footswitch starts recording, the next closes the loop and starts playback, and further layers go on top via overdub. Everything depends on that second press landing exactly on the beat, because a loop ten milliseconds too long drifts audibly out of time with every pass. Better units can therefore quantize the loop length or follow an external MIDI clock.
Before digital, it was done with tape loops and long tape delays. Digital pedals took over in the 1990s, and Boss's RC series turned live looping into a stage genre of its own — Ed Sheeran built a career on one. Inside a DAW, Ableton Live's Session View is effectively a looper with thousands of tracks.