It's like an internship that people do when they are thinking about becoming teachers. Usually it is a university student who is just finishing up their coursework on teaching, and they spend ~8 weeks or so working under an existing, credentialed classroom teacher in a public school. It lets them practice working with real students under more experienced supervision.
They are not paid for it, and it's a required part of their training.
They are not paid for it, and it's a required part of their training.