Prerequisites are courses that need to be completed before taking another class. This prevents you from taking an advanced class without having the knowledge to complete it.
When viewing class descriptions, prerequisites are usually listed at the bottom.
The example below shows that CIT 348 requires that you have completed CIT214 and STA261 or STA368 or ISA 205 (any one of those three) before being allowed to register.
Image Source: https://bulletin.miamioh.edu/courses-instruction/cit/
It is sometimes permitted to take prerequisite courses concurrently (at the same time) with the higher-level class. Sometimes the prerequisite may not be required at all if you have a sufficient demonstration of the related knowledge.
However, you will get an error blocking you from registering through the normal process in Banner Web when a prerequisite course is not found in your student records.
Instead, you will need to submit a Registration Override Request (ROR). Once this is approved by the instructor or department, the class will be added to your registration for the semester. If the request isn't approved within a few days, reach out to the instructor to follow up.