The management of your tenancy is split into three levels. These are Organisation, Site and Group
The organisation is the topmost level. All devices across your sites are viewable here.
When your tenancy is set-up it will contain one site, it is possible to add further sites as per your requirements. Multiple sites are useful when managing more than one school from a single organisation (such as a MAT), where you would typically setup one site per school. Devices are only able to be a member of one site in your organisation.
Within each site it is possible to have multiple groups. Groups are often used to represent all the devices in one room or a particular subset. Devices can be a member of multiple groups within a site.