Mortgage rates will fluctuate up and down based on the market. Rates can change daily, so you don't know what your rate will be until you lock it. By locking a rate, your lender is guaranteeing you that rate for a specific amount of time, typically 30-45 days. Once you lock your rate, it doesn’t matter if the market fluctuates. If you locked at 4 percent and rates go up, you still get the 4 percent. The risk is on the lender when you lock a rate.  However, if rates go lower, you don’t get the benefits of a lower rate.