Outside of the time specified, everything results back to default (or non-group policy mode if you will).
So if for example, you block access to Facebook as a default setting.
However you want to allow it for 1 hour (lunch break), so you create that group policy that basically mirrors your default, with the exception of allowing Facebook. You allow it only during a 1 hour lunch period, so then the device will be able access Facebook during that time period. When the schedule ends, the group policy is no longer applied and they will no longer be able to access Facebook because your 'default' settings have it blocked.