To put it simply, grants do not have to be paid back, but loans do have to be paid back. Grants are funds designated by the U.S. Department of Education to aid students in their pursuit of higher education. 

A subsidized Stafford Loan is a federal student loan where the government covers the interest that accrues while the borrower is in school at least half-time, during the grace period, and during deferment periods. However, once a student either falls out of half-time enrollment, drops in academic performance or finishes their education, they have to begin to pay back the loan and pay the previously agreed interest rate on the loan.