On this tab, you can see a list of roles defined for the group. These roles are applicable only in the context of the group. Don't confuse them with website roles, which can be set in Site Manager -> Administration -> Roles.
Roles in the list can be Edited () and Deleted (). If you click the New role link above the list, you can define a new role for the group.
More resources can be found at Developer's Guide -> Modules -> Groups -> Overview.