As opposed to a fixed rate mortgage where all the payments are the same over the mortgage lifetime, adjustable rate mortgage means your payments will vary over time. The mortgage payments will fluctuate according to the interest rates. When the interest rate is adjusted your mortgage payments will be adjusted accordingly.
Generally adjustable rate mortgages will require you to pay a fixed rate for the first year, this rate is usually very low, and then vary from the second year onwards according to the interest rates. If the interest rates go down, your payments will also go down and if the rates go up, your payment amount will also increase accordingly. The benefit of getting a variable rate on your mortgage is that you have the ability to take advantage of times when the interest rates go down. Since adjustable rate mortgages have a fixed period and variable period you may find that your interest is adjusted when the rates go up causing you to pay more in the long run. Therefore it is best to hire a professional to obtain advice regarding the adjustable mortgage you wish to get. They will help you calculate and compare the total amount, list out the benefits of each type of mortgage and help you to make the best of your money.
As with any deal you should always thoroughly read through the contract and ensure that you are fully aware of all the clauses set by the lending institution.