A precondition is a prerequisite. It’s the thing that has to happen before something else happens. For example, as a precondition to getting your allowance, you might have to give the dog a hairdo once a week.
Precondition is often used in the negative – when world leaders meet to discuss something, they might ask for no preconditions. Clean slate. On the other hand, good grades are sometimes a precondition for joining the school basketball team. Applying for a new job can be frustrating when you realize that experience is a precondition, but you're not sure how to get the experience without getting a job. When it's a verb, precondition means to prepare something (or someone).