Gluten is a protein found primarily in wheat, which is responsible for the elasticity of kneaded dough which allows it to be leavened, as well as the "chewiness" of baked products like bagels. It can be extracted and cooked to provide a very convincing mock meat, due to its "stringy" texture.
Gluten can be extracted by hand via a time consuming process, or purchased in powdered form from health food stores.
A popular gluten mock-meat product is seitan, which is gluten that's been cooked in a broth to give it a flavor similar to that of beef.
Gluten is used heavily in Chinese Buddhist cuisine.