A Lock-in Period is a pre-determined duration of time (post the issuance of shares, purchase of an asset or investment in a scheme) in which the customers or investors are forbidden from making any sale, transfer or withdrawal transaction.
All investments do not have a lock-in period.

For example, in case of mutual funds, one can exit anytime, except in ELSS Funds. ELSS Funds have a lock-in period of 3 years.