An ARM is a mortgage that typically has a fixed interest rate for a set number of years. After that, the mortgage rate changes periodically until the loan is paid off.
ARMs are usually expressed in two numbers. The first number is the number of years you'll pay the fixed rate (also called the initial fixed-rate period or introductory period). The second number is the adjustment period (or adjustable-rate period). The second number indicates how often your interest rate will adjust after the introductory period.