Prepayment means paying an extra lumpsum toward your loan's outstanding principal, over and above your regular scheduled EMIs, whenever you have surplus funds available — say, from a bonus, tax refund, or savings. This extra amount goes directly toward reducing the principal you owe, which in turn reduces the total interest you'll pay over the remaining life of the loan.
Because interest is calculated on your outstanding principal, even a single well-timed prepayment, especially early in a long loan tenure, can shave off a meaningful chunk of total interest and shorten your loan tenure. Many banks allow you to choose whether a prepayment reduces your EMI amount (keeping the tenure the same) or reduces your tenure (keeping the EMI the same) — reducing the tenure while keeping EMI constant generally saves more interest overall.