Both repayment and prepayment refer to paying back the debt to a creditor. However, the timing is different.
Repayment is the extinguishment of an the obligation to repay the loan according to the original schedule. Loans can be repaid in various ways, either in regular instalments or in one lump sum upon maturity.
Prepayment is the repayment of the loan before its original maturity date. In most cases, it is subject to special conditions determined in the loan contract.