When something precludes another thing, it makes that second thing impossible or prevents it from happening. The word suggests a firm barrier: not just difficulty, but actual prevention.

Heavy rain might preclude an outdoor concert, forcing organizers to cancel or move inside. A serious injury could preclude an athlete from competing in the championship. If you're grounded, that precludes going to your friend's party. When a rule says “no food or drink in the library,” it precludes bringing your snack along while you study.

Preclude is stronger than words like prevent or hinder: it suggests that something has been ruled out entirely, not just made harder. A conflict in your schedule might prevent you from attending two events, but if something precludes participation, you cannot participate at all, no matter how hard you try.