Mutability means the quality of being changeable. Caterpillars, on their way to becoming butterflies, display a great deal of mutability.
An easy way to remember mutability is to think about a word it sounds like, mutant. A mutant is someone who has been changed, irrevocably, so mutability is the ability to change. It's the mutability of the Internet that makes it so wonderful and confusing at the same time. You might tell your friend who was sweet and kind yesterday, but is mean and awful today, that you wish she didn't have such a tendency to mutability.