It usually means that you can sit in the lectures, but that none of your work will be graded/marked and you won't get any credit for it. Some universities have officially forms that record the fact that you audited a course, and some just do it through personal consultation with the instructor.

During my first year of grad school, I audited a codes and cryptography course my senior-year undergrad friends were taking. I sat through the crypto part and then bailed on the codes. It was nice. I know a little about RSA and Elliptic Curves now. I got to hangout with my friends, and I didn't have to do any work. I think I'm a better educated person for it, but it doesn't really affect my career.

It usually means that you can sit in the lectures, but that none of your work will be graded/marked and you won't get any credit for it. Some universities have official forms that record the fact that you audited a course. Others just do it through personal consultation with the instructor.

During my first year of grad school, I audited a codes and cryptography course my senior-year undergrad friends were taking. I sat through the crypto part and then bailed on the codes. It was nice. I know a little about RSA and Elliptic Curves now. I got to hangout with my friends and I didn't have to do any work. I think I'm a better educated person for it, but it doesn't really affect my career.