what is the function of pepsin?

Pepsin is a digestive protease (EC 3.4.23.1) released by the chief cells in the stomach that functions to degrade food proteins into peptides.
According to American Heritage Dictionary, pepsin derives from the Greek word pepsis, meaning digestion (peptein: to digest).
Pepsin was discovered by Theodor Schwann in 1836. It was the first animal enzyme to be discovered.

Precursor
Pepsin is expressed as a pro-form zymogen, pepsinogen, whose primary structure has an additional 44 amino acids.
In the stomach, chief cells release pepsinogen. This zymogen is activated by hydrochloric acid (HCl), which is released from parietal cells in the stomach lining. The hormone gastrin and the vagus nerve trigger the release of both pepsinogen and HCl from the stomach lining when food is ingested. Hydrochloric acid creates an acidic environment which allows pepsinogen to unfold and cleave itself in an autocatalytic fashion, thereby generating pepsin (the active form). Pepsin cleaves the 44 amino acids from pepsinogen to create more pepsin. Pepsin will digest up to 20% of ingested carbon bonds by cleaving preferentially after the N-terminal of aromatic amino acids such as phenylalanine and tyrosine. It will not cleave at bonds containing valine, alanine, or glycine. Peptides may be further digested by other proteases (in the duodenum) and eventually absorbed by the body.
Pepsin is stored as pepsinogen so it will only be released when needed, and does not digest the body's own proteins in the stomach's lining.
Pepsin functions best in acidic environments because it is found in an acidic environment, particularly those in a pH of 1.5 to 2.
Pepsin is said to have an optimum temperature between 37C and 42C in humans.