Some institutions use credits system in their study.
It is a measure of a value the course has for a student. 

For example, if you study programming, Math course has more value for you than e.g. PT.
Thus, Math may have Credit 3, whereas PT has Credit 1.

Credit should always be more than 0. 
That is why it is set to 1 by default.  

If your education process does not involve credit system, please leave your credit amount (1) unchanged.
Otherwise your GPA will be calculated wrong.

To change the number of credits in iStudiez Pro go to the Planner > Semester > Course > Grades > Credits - set the number of credits you require.