Prerequisites are requirements students must satisfy before registering for a particular course. For example, NMU's HS 200 Historical Thinking and Writing course has a prerequisite of EN 111 College Composition with a grade of "C" or better.
To make sure you schedule all your courses in the right order to accommodate all the prerequisites is to get a copy of the courses and degrees catalog or bulletin published by your university These guidelines provide a list of required classes, class descriptions and any other requirements you might need to fulfill (e.g. senior thesis) in order to graduate. More and more schools are putting their bulletins online (like NMU's undergraduate bulletin) making it easier to understand program requirements before enrolling.
Course guidelines are revised yearly, so make sure to get an updated version. Even though degree requirements change over time, most colleges will have you stick to the program plan listed the year you start.