A prerequisite is a paper that must be passed before taking another paper or course. Some papers have more than one prerequisite.
If you have not completed a prerequisite for a certain paper you will not normally be permitted to enrol in that paper.
For example, it is not normally possible to enrol in ZOOL222 without having passed CELS191 and (BIOL 112 or (HUBS 191 or HUBS 192 with at least a B pass)).
Corequisites are papers that must be taken concurrently with another paper (unless it has already been passed). For example students are expected to take both EDUC240 and EDPR240 at the same time.
To check if a paper you want to add has a corequisite or prerequisite see Subjects and Papers at Otago