"User is idle" means that a screen saver is running on the machine or the machine is locked.
Example
Run the backup on the machine every day at 21:00, preferably when the user is idle. If the user is still active by 23:00, run the backup anyway.
As a result,
(1) If the user becomes idle before 21:00, the backup will start at 21:00.
(2) If the user becomes idle between 21:00 and 23:00, the backup will start immediately after the user becomes idle.
(3) If the user is still active at 23:00, the backup will start at 23:00.