Some people belong to more than one tenancy: a consultant working for several firms, or a group with separate companies.
Switch tenancy
Use the tenancy switcher in the top bar and pick the one you want. Conform reloads into it. Everything you see then belongs to that tenancy: its data, your roles there, even your Quick Nav list.
The Tenancy Manager
Tenancies → Tenancy Manager lists every tenancy you belong to. From here you can switch, open the settings of the current one, or start a new tenancy.
Start a new tenancy
Start your own subscription begins a new tenancy of your own. See Setting up a new tenancy.