The Lock-in period is a time wherein the investors can't redeem or sell their units without paying a charge (Exit load). An exit load of 1% is usually charged if Equity funds are withdrawn before a year. However, investors may sell their funds anytime after the lock-in period without any charge.
For instance, in the case of an ELSS plan or tax saving mutual funds, a lock-in period of 3 years is applicable during which an investor can't exit the fund at all.