Obtaining the lowest mortgage interest rate is crucial for prospective homeowners, as lower rates lead to reduced monthly payments. However, advertised rates may not be available by the time of closing due to daily fluctuations. A mortgage rate lock guarantees a specific rate, protecting borrowers from rising rates but potentially preventing them from benefiting from falling rates. While some lenders charge for rate locks, others offer them for free for shorter periods. It's essential to shop around and secure rate locks in writing to avoid unexpected increases.