“I'm just auditing it, so it doesn't count toward my major.”
You're a university student studying Business. You're taking one art class this semester just for fun. But you're only attending the class, not getting a grade for it. Another student in the class finds out that you're studying business, so she asks why you're taking this class. This is your response.
I'm just auditing it, so it doesn't count toward my major.
When you take classes, you usually take them for credit. This means that you get a grade and the class counts toward your degree. But in most colleges and universities, you can also "audit" a class. This means that you just come to the class and listen to the lectures, but you don't get a grade and the class doesn't count toward your degree.
When you have a specific goal, something that "counts toward" that goal is officially recognized as a "point" that brings you closer to the goal. In many situations, there are some actions that "count toward" your goal and others that do not. For example, if you're a salesperson you may have a sales target. If you sell to new clients, the amount is added to your total sales amount, but things you sell to previous clients are not added. You would say:
Sales to existing clients don't count toward my weekly sales targets.
Your major is the field that you get a degree in. Example majors are Biology, History, Psychology, Engineering, Computer Science, Business, etc. Students who are meeting each other for the first time often ask:
What's your major?
"Major" is usually just used to talk about a person's undergraduate college or university degree.