Basically an older student helping teach the younger ones as a parttime job. Generally involves a lot of crappy work like supervising labwork, helping out with grading and answering the same question 18 times.
I bet you'll NEVER forget any of that material ever your whole life. You'll be on your deathbed senile can't remember your name and can still spout that shit out.
In my personal experience, it usually means doing 80% of the professor's work for minimum wage pay so the $140k/yr prof can fuck off and go brown-nose the school Board of Trustees all term.