A teaching assistant (TA) is often a graduate or upper-level student who may aid in lab courses or seminars, provide tutoring, and/or grade course assignments. TAs work closely with the professor of a course and help students succeed in the class. Don't be afraid to reach out to the TA for additional support if you're confused about a topic!
If you really enjoy a class and receive a high grade, you can possibly become a TA as an undergraduate student. TA positions are sometimes posted as work-study jobs.