Within the Ember door, you will see a little hole. This is covered by the window cover.
As the name suggests, this allows you to peek inside your oven without removing the door. This saves you losing temperature but also avoids an influx of oxygen which could cause your temperature to spike.
The window cover simply screws in with one screw, then you just need to attach the handle.
You can also use the window cover to reduce the temperature of your oven slowly if you find that it is creeping up!