The grading system in the US colleges / university follows a GPA (Grade Point Average) system.
If you come from a country that use a percentage system, or any other system other than the GPA system, this section will help you to have a better understanding on how the GPA system works.
What does a credit / non-credit class means?
If you take a credit class, by the end of the class you will receive a grade based on your performance in the class (A, B, C, D, F, I, W). This grade will appear on your academic transcript and counted towards your GPA.
On the other hand, if you take a non-credit class, after you are done with the class, on your transcript it will appear "credit" if you passed the class and "no credit" if you didn't pass the class. Therefore, it cannot be included to calculate your GPA