A group is a set of users with the same access privileges.
For example, your account might have one site, “Maple Street Office,” which has two doors. If we assume that all employees require the same level of access to both doors, then a single group, “Maple Street Staff,” would be sufficient.
Or, your account might have one site, “Elm Street Diner,” which has three doors. If we say that waiters require access to “Front Door” and “Back Door” but not “Office Door,” while managers require access to all three doors, then it would make sense to create two groups, one called “Elm Street Waiters” and one called “Elm Street Managers.”
Or, your account might have two sites, “Miami Store” and “Orlando Store,” which have two doors each. At each store, staff members require access to “Front Door,” while managers require access to “Front Door” and “Stock Room.” In this case, you probably would want to create four groups: “Miami Staff,” “Miami Managers,” “Orlando Staff” and “Orlando Managers.”