Loopers are delay lines with extremely long delay times, effectively allowing you to record part of a performance and create a loop from it. One of the first people to use this method was the legendary bass guitarist Jaco Pastorius, during his time with the band Weather Report.
Using a looping delay pedal, its possible to pre-record riffs or patterns and then play solos or other parts over the top. This can be great for solo performers! Some models allow you to store multiple loops and layer them at will.