Peristalsis is the group of muscle contractions that advance the contents of a hollow organ (gastrointestinal tract, blood vessel, etc).
Peristalsis is caused by progression of a peristaltic wave produced by unconscious sequential contraction of neighbouring longitudinal or circular fibre smooth muscles.
The mechanism is responsible for gastrointestinal transit and blood circulation in organisms that do not have a cardiac organ.
Progression of the peristaltic wave in the antrum of the stomach. © Gastrolab.net, posted on YouTube by Endoscopyvideos