A configuration management tool tracks the state of system resources to ensure that we don't attempt to reapply the same configuration repeatably. If the package is installed - do not attempt to install it, if the service is running, do not try to start it again.
The point is that once the desired state of a system is reached, re-applying the configuration instruction always has the same result.
An operation is idempotent, if the result of performing it once, is exactly the same as the result of performing it repeatedly without any intervening actions.