This is the Client App details page. Use this page to modify the Client App'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 Client App can be registered with the Gateway. In particular, the Client App must have at least one API Contract (see the "Contracts" tab).
The 'Contracts' tab is where you can manage all of the API Contracts for this Client App. A API Contract is simply a link between this Client App and a provided API the the Client App consumes.
The 'Policies' tab allows you to manage the Client-level policies that should be applied whenever a request is made to any API consumed by this Client App.
The 'APIs' tab lists all of the APIs consumed by this Client App, including showing the API Key and live endpoint for each. Go here if you want to figure out how to invoke an API once you have a Contract for it.
The 'Change Log' tab shows a history of all the changes made to the Client App. Essentially it is an audit log.