A prerequisite is a required prior condition. If something is required in advance of something else, like if you have to take a beginning Spanish class before signing up for Spanish II, then it’s a prerequisite.
Add pre meaning "before" to require and you have something that's "required before." (That's helpful when trying to spell prerequisite!) Prerequisites to be able to graduate from college can seem like a pain, but most schools offer some fun ways to fulfill them. For example, you can often meet the physical education prerequisite by taking bowling. Prerequisites aren't just for school: “The most called-upon prerequisite of a friend is an accessible ear,” said Maya Angelou, the American poet.