A rate "lock" or "commitment" is a promise from the lender to hold a particular interest rate and a specific number of points for you for a specified period of time while your application is processed. This means your interest rate won't go up during the application process.
Although there are various lengths of rate lock periods (from 15 to 60 days), the longer spans are usually more expensive. You can get a longer period for your lock, but in making this choice, will probably have a higher rate than you would have with a shorter rate lock span of time
There are more ways to get a low rate, in addition to going with a shorter rate lock period. A bigger down payment will result in a better interest rate, since you will have a good deal of equity at the start. You could opt to pay points to bring down your interest rate over the loan term, meaning you pay more initially. One strategy that makes financial sense for many people is to pay points to bring the rate down over the life of the loan. You will pay more initially, but you'll save money in the long run.
Do you have a question regarding a mortgage program?