To defray means to pay for something, usually by covering part or all of a cost. If your school is raising money to defray the cost of new playground equipment, they're trying to collect enough to pay for it so the school doesn't have to cover the whole bill alone.

You'll almost always see defray paired with words like “cost,” “expense,” or “expenses.” It's a slightly formal word that shows up when people talk about how bills get paid, especially when the amount is large or shared among several people. A bake sale might defray the expenses of a class field trip, or a scholarship might defray the cost of college tuition.