Here it means 2 classes that must be taken at the same time. You need to take them both simultaneously in the same semester. Pre-requisite means that you need to take the certain pre-requisite course to get into higher level courses/declare a major etc.
A course to be taken at the same time as another specified course, or courses, as designated by the department. A corequisite course may also be completed before taking the course or courses with which it is paired. --- taken from google.
Apparently, there is such a thing. And after looking at some of my classes, some of them do list co-reqs... but I had already taken the classes.