From Electro Harmonix;

Capture a frozen moment and turn it into a tonally unique sonic
foundation. The Freeze Sound Retainer delivers infinite sustain of any
note or chord at the press of a momentary footswitch. Release the
footswitch and you are again sample ready. Three selectable decay rates,
including a latch mode, guarantee liquid, smooth tonal transitions.
Hook the Freeze up with your favorite pedals for a sonic collage that
will be unlike anything you have ever heard. Its like adding an extra
musician to the band.