The overview tab shows the an overview of the current map location. The map view corresponds to the purple bounding box visible on the overview.
You can click and drag the bounding box to move the map around.
If you have rotated the map or have map rotation set through GPS the bounding box will automatically rotate to in sync with the map view. The brighter edge of the bounding box signifies the top of the map view.
Show entire map
Check this option if you want to force the overview to always display the whole map. If not selected you can choose the scale of the overview in relation to the current map view. On the bottom right of the tab you can the current overview scale.
Show all views
If you have more than one map view open you can check this option to display the bounding boxes of all of them. Note that the View:Overview will be in relation to the map view with the highest zoom.