Locking in a rate means that, so long as you close within the lock period, you have certainty about what the interest rate will be. Without a lock, rates can change. You may end up with a more expensive loan than you originally planned for; sometimes the rate could increase so much you can no longer qualify for the loan. On the other hand, rates can also go down, and if you lock the rate you won’t benefit from any decrease. Overall, whether to lock the rate will depend on your finances and on how comfortable you are with the possibility of rate changes.