An account must have at least one community.
Think of communities as organic groups inside your account. For example: Corporate, Sports, etc.
Each community will hold a set of channels (i.e. social channels like Facebook and Twitter; or also custom channels like TV, radio, newspaper). It will also have one or more projects, inside which content is organized and scheduled in the form of campaigns and posts.
You can add members to your account, giving access to every community of it or – if you want more granular access – it's possible to add a member just to specific communities, so other communities will be not visible to him.