Pepsin is a kind of enzyme. It acts on proteins in the stomach, which has a low pH because of Hydrochloric acid in it. It works best at a pH of about 1.5. It will not work at a pH above 6.
It was the first animal enzyme to be discovered. Theodor Schwann discovered it in 1836.
Pepsin is used to make cheese. The brand name Pepsi comes from pepsin.