What does the idempotency of HTTP methods mean and why do some of them have this property?

HTTP method idempotency means that multiple executions of the same request will result in the same outcome on the server, without additional side effects after the first call.

For example, the PUT method is usually idempotent because it updates a resource to a specific state. Sending multiple identical PUT requests will not change the resource's state after the first update.

Methods GET, PUT, DELETE are considered idempotent because repeated requests do not further change the server's state. POST is not idempotent because each request typically creates a new resource or triggers an action with a side effect.

Idempotency is important for reliability and retrying requests, for example, in case of network errors or repeated attempts.