Pre-requisites means that you are required to take a pre-requisite course first before taking the next level course. For example, you need to complete GSR 101 and GSR 102 before taking GSR 150.  Co-requisites allows you to take a co-requisite course if you are taking the pre-requisite course at the same time.  For example, you can be enrolled in EDU 323 as long as you are taking EDU 311 concurrently.