The Explorer tree contains database landscapes and user management functions. Most Database Studio functions are available via the context menu in the Explorer tree.
Note that depending on your configuration, nodes further down in the hierarchy may be shown when opening Database Studio.
World
Local
My Landscape
Servers
Local
<database_1>
...
<database_n>
Other database computers
User-defined database landscapes
User Groups
Users
Roles
Permissions
More information: Managing Database Landscapes