Say this if it’s possible to change the fit of something.
For example, imagine that you’re trying on a ring at a jewelry store, but it’s a little too small for your finger. The saleswoman takes the ring and shows you that it can be pulled apart to become larger:
Here you go. It’s adjustable.
Other things that are "adjustable" include settings on a machine, like the volume, brightness, or color balance of a TV.
In general, words that end in "-able" mean "able to be ___ed". Some other examples: