Students earn one semester hour of credit for each hour a class meets per week for a semester. If a class meets only one hour per week for the whole semester, it would be a 1 credit class. Most classes meet for 3 hours per week for the semester and are there for 3 credits.
Some classes are labs and labs count differently. For every 2 hours in the lab students earn 1 semester hour of credit. Part of the reason is that students rarely have any additional out of class study time for a lab class (since they need the resources of the lab).
So, a 15 credit hour semester schedule would mean a student is in the classroom 15 hours per week. Don't think "Wow! That's easy!" though. For every hour of class, students should plan 2-3 hours out of class in studying and assignments. Some weeks it will be less, some more. In some classes students won't need any studying - in others they will live-sleep-eat that class work.
Students should expect, in order to achieve a respectable GPA, to spend 30-40 hours per week on school (class, homework, assignments) on a full-time schedule. Academic research has proven that as students reduce those hours, their GPA will fall.