What is an audit?
Signing up to attend and learn in a class without having to do the assignments, but also without earning credit. Audit comes from Latin words about “hearing,” so to audit a class is to “hear” it, but not to do the required work.
Typically attendance in a specific percentage of the class sessions is required for the audit to be considered acceptable. This is usually marked somehow on your transcript, such as with SA for “successful audit.”