A group allows for organization of apps in a tree-like hierarchy. Groups can represent physical spaces, virtual spaces, objects, etc. They are structured in a tree hierarchy so you can describe more complex structures with them.
An example of a hierarchy of groups could be:
> University Campus > Jamrich Building > 2nd Floor > Classroom 251
A group has zero or more apps, which define common interactions relative to that group. For example, an app requesting more printer paper makes sense for an office supply room group but does not make sense for a bathroom group.