What's a "student teacher"?

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.

https://en.wikipedia.org/wiki/Student_teacher

A student in an Education program doing their teaching experience at a school, before they graduate with a degree in Education.

A student in a masters or bachelor’s program who teaches or assists a certified teacher.

They are in the process of getting their accreditation.