A mortgage rate-lock agreement means a lender offers you an interest rate for a period of time, regardless of market changes, possibly for a fee.
A shorter rate-lock term usually means a lower interest rate.
If the lock expires before you close on a house, the lender might extend it for a fee. If not, you will get the then-current interest rate.
If interest rates fall before closing and you have a float-down provision, you’ll get the lower rate.
If rates rise before you close, your interest rate might be higher than the one you locked in.
So be sure to choose a rate-lock term based on your anticipated closing date.