An adjustable rate mortgage is a mortgage where the interest rate is adjusted according to the present level. This means that your rate will be adjusted up or down depending on the general situation on the market. Adjustable loans are the norm in many countries and in these countries they are referred to simply as mortgages.
The main thing that is interesting with an adjustable rate mortgage from the perspective of the lender is that you might get a cheaper loan if the interest rates fall during the loan period. But this isn't a certainty as the rates just as well might go up and then your loan will be more expensive instead.
If you think that the interest rates are high in general at the moment and will probably drop in the future, it will probably be a good idea to use the adjustable rate mortgage, since the fixed rate you would get when choosing a fixed rate mortgage will probably be a bit too high. If you feel that the interest rates are hitting rock bottom right now, it is certainly better to go for a fixed rate since you will probably be getting your fixed rate cheaper than ever and the adjustable rate will surely go up again in the near future.
A mortgage is a big decision and it is connected to large costs in the future. The choices you make now will probably effect your economy and your life for many years ahead and it is very important to take a moment to think things over. If you are not familiar with interest rates and dont know if they are high or low right now, its best to talk to someone or look around some more on the Internet.