The "Board" widget is a part of the "Board" module.
The instructions of how to use widgets are given in the "Home -> Widgets" section. This widget allows you to monitor the actual information of the "Board" module, if it is installed on the "Home" tab. It also allows you to comfortably navigate through that information. It means that you will see in the special part of the "Home" tab the list of the recently changed topics of all Board categories. You will also be able to enter those topics by pressing with your mouse on their subjects.