This is a course (or courses) that you must complete before you can enroll in a given course.
If a course is specified as a prerequisite, you must achieve a passing grade in that prerequisite before you can take the course (unless a specific grade is indicated, in which case you need to achieve at least that grade e.g. some courses may indicate you must achieve a grade of 80% or higher in the pre-requisite course).
Pre-requisites are listed in the course description when registering on your Student Centre and in the Academic Calendars course descriptions.
University Of Calgary Prerequisite