Types of Prerequisites
There are three major types of prerequisites.
School year: Especially in colleges, some courses require you to be at a certain grade level and/or number of credit hours before you can register. For example, a particularly tough science class might say “junior/senior required," meaning that you have to be a junior or senior to sign up for it.
A previous class: Some courses require you to have taken another class first, or even a couple of classes. Cognitive Psychology might require that you first take Psychology 101, and a class like Photography 2 will almost always require that you first complete Photography 1. Calculus 3 likely requires Calculus 2, which in turn will require Calculus 1. Generally you will see higher-level, tougher classes that have lower-level classes as prerequisites. Pay special attention in college to the general education program (if you have one) since there are often prerequisites that force you to take those classes in a certain order.
Grade in a previous class: When a previous class is a prerequisite, it means you need to have passed that course. But other higher-level courses require a certain grade in a related class, not just a pass. Spanish 2 might require a B or higher in Spanish 1, and Creative Writing might require a C or higher in English 101. If you don’t have the grade you need, you may have the option to retake the lower-level class to try and achieve a better grade.