Global dashboards are available at the instance level, and can be accessed through the menu on the left when you're in a global context (I.E. not drilled into a project or other resource). One of the global dashboards is set as your home page. Whatever your context, you can always return to your home page using the "Dashboards" link in the top left corner of the SonarQube interface.
Each box on this page is called a widget. Any widget can be added to a global dashboard. Thus, any kinds of information from a project or from the SonarQube instance can be displayed at will.
Global Dashboard Shipped with SonarQube: Home
This dashboard is fully editable by SonarQube administrators.
If you're logged in, you see an additional widget:
Default dashboards are only editable by SonarQube administrators, but each user can create private dashboards. For more on dashboard editing, see customizing dashboards.