coursework - What does “auditing a course” mean?

I am visiting an American university for a year. In my invitation letter and further instructions, it is written that I can audit any course that I want (subject to the agreement of its professor).

Since it is not common in my country, I have no idea what this auditing process is. What should I do?

Is it an official process with official report, or do I simply attend the class like students?