In WebSched Pre-requisites has a specific, and not standard, meaning.
If your school is semestered (or multiple terms) and a student is requesting both Math 10 and Math 11. Then obviously Math 10 should be taken in the first semester and Math 11 in the second. In WebSched terms, Math 10 is a pre-requisite for Math 11.
If you designate course A as the pre-requisite for course B, then the AutoScheduler will ensure that for all students requesting both courses A will be scheduled before B.