This is the period during which you are contractually obliged to stay with the lender. This means you are not allowed to make partial/full prepayment of your home loan. The lock-in period comes as a condition for receiving a particular fixed rate, discount, and other perks.
Refinancing your loan is considered a full prepayment of your existing home loan. If you choose to refinance your home loan before the lock-in period ends, you may have to pay hefty prepayment penalties, typically 1.5% of your outstanding principal.