Proline is a building block your body uses to make collagen, the main structural protein in skin, hair, and joints. Collagen needs a steady supply of proline to form properly, so getting enough matters for connective tissue throughout the body.
Some people pair proline with glycine and vitamin C to aid collagen formation. Others take it to support post-exercise recovery, though the data behind that use is thin.
High doses may cause mild stomach discomfort, but side effects are otherwise uncommon.