What does “auditing” mean?

I received this inquiry:

What is your definition of auditing a class?  I am trying to decide between auditing Henle 1 and “I already own Visual Latin”.

Thank you!

 Here is my reply:

I personally grade the work of my students.   This, however, is time-consuming.   For those who do not need this type of grading, I set up the “auditing” system. Students “auditing” the class will not submit their work to me. Students “auditing” the class will still have access to the previously viewed class, the online quizzes, and even, if they are available, the live classes.