Like reverb, delay is a pedalboard essential, no matter what type of music you play. Delay pedals take a played phrase and repeat it, sometimes referred to as reflections. These reflections add depth to the music your playing and in some cases can become an recognizable instrument in your favorite songs, just ask Pink Floyd. The intensity, rate, and number of those reflections are controlled by the level, time, and feedback knobs on the pedal. And that's just to start.
Delay pedals have come a long way in recent years and some now incorporate additional parameters, such multiple delay times, filters, and modulation.