When you choose to lock-in your interest rate, you are guaranteed that rate through the expiration date. Rates can be locked on a 15, 30, 45 or 60 term. The longer the lock is guaranteed, the more risk the lender takes and therefore the higher your rate.
The benefit is the security of knowing the interest rate is locked in if interest rates should increase. The down side is if interest rates fall, you are locked in at the higher interest rate.
When floating the interest rate, you take the risk of interest rates increasing before you lock-in. The benefit, however, is if interest rates go down, you would have the option of a lower interest rate than if you had locked in previously.
The decision of whether to lock-in or not is a personal choice. We can give you advice, but ultimately it is your decision on the amount of risk you are willing to take.