What is the substrate of pepsin?

The substrate is proteins, and the products are peptides.

Pepsin is a protease, i.e. an enzyme that catalyzes the breakdown of proteins. In humans, it is secreted by certain cells (called chief cells) in the lining of the stomach.

Specifically, pepsin breaks the peptide bonds linking amino acids in the chains found in proteins. Pepsin cannot break all peptide bonds, however. It is particularly efficient at hydrolyzing the bonds on the C-terminal side of tyrosine, phenylalanine, and tryptophan. It does not attack bonds involving glycine, alanine, or valine residues.

So you can think of pepsin as being like someone taking a pair of scissors to a long piece of string, and cutting in certain places. The result of peptic activity is shorter chains, called peptides.

These peptides are later further digested by other enzymes secreted by the pancreas and small intestine. The final result is free amino acids, which can be absorbed by the villi of the small intestine.