This is the application details page. Use this page to modify the Application's meta-data, policies, and contracts. There is no need to follow the tabs in order, but note that you will need to fill out a minimum amount of data before the Application can be registered with the Gateway. In particular, the Application must have at least one Service Contract (see the "Contracts" tab).
The 'Contracts' tab is where you can manage all of the Service Contracts for this Application. A Service Contract is simply a link between this Application and a provided Service the the Application consumes.
The 'Policies' tab allows you to manage the Application-level policies that should be applied whenever a request is made to any Service consumed by this Application.
The 'APIs' tab lists all of the Services consumed by this Application, including showing the API Key and live endpoint for each. Go here if you want to figure out how to invoke a Service once you have a Contract for it.
The 'Activity' tab shows a history of all the changes made to the Application. Essentially it is an audit log.