A 5/1 arm loan is actually an adjustable-rate long-term mortgage. If talking about the meaning of '5' and '1' figures, it is as follows. '5' means five years during which you have a fixed interest rate, and '1' means one year, which states frequency of changing of your interest rate after the expiration of the first five years. Sometimes these changes might be significant, so you have to start paying way more than before.