Got asked this question the other day and my mind pulled a blank on me. However an idempotent method is really straight forward. It’s just a method that no matter how many times its called the result state will always be the same, you can call the method many times without concern.
A good example of an idempotent method is the close() method on a data connection, you call it once, it closes, you call it a second time? Still closed, a third time? You get the idea.
This answer on the question of “What is idempotence?” on Stackoverflow explains it really well: http://stackoverflow.com/a/19320202/1445352