An adjustable rate mortgage, or an "ARM" as they are commonly called, is a loan type which offers a lower initial interest rate than most fixed rate loans. The trade off is the interest rate can change periodically, usually in relation to an index, and the monthly payment will go up or down accordingly.
Against the advantage of the lower payment at the beginning of the loan, you should weigh the risk an increase in interest rates would lead to higher monthly payments in the future. It is a trade-off. You get a lower rate with an ARM in exchange for assuming more risk.