Lock in period is the time period from the date of investment during which you cannot redeem and exit the investment. Some mutual funds have a lock-in period.
For example, ELSS funds have a 3-year lock in period. It is to ensure that investors stay in the fund for a certain period and benefit from long term equity holding and growth. Lock-in period also ensures that the fund manager has a stable corpus to take long term investment decisions to even out short term fluctuations in the market.