Idempotency means that doing the same operation more than once has the same lasting effect as doing it once. It is useful when an app retries a request because the internet connection dropped.
For example, setting a profile's color to blue twice should still leave it blue—not create two profiles. The repeated request does not add another change.
Not every action is idempotent. Sending money twice could charge twice unless the service is designed to recognize the repeated request. The important question is what lasting change happens when an operation is repeated.