An adjustable-rate mortgage, or ARM, has an introductory interest rate that lasts a set period of time and adjusts annually thereafter for the remaining time period. After the set time period your interest rate will change and so will your monthly payment.
The initial interest rates for adjustable rate mortgages are normally lower than a fixed rate mortgage, which in turn means your monthly payment is lower. If you only plan to stay in your home for a short period of time, an ARM loan might be advantageous to you because you plan on moving or selling your home before your initial mortgage rate adjusts.