Just like the Internet, resources wouldn't be very interesting if they weren't linked to other resources; for example, a repository has packages, a package is inside a repository, etc. When a resource is linked to another, the API uses hypermedia links to represent this. A hypermedia link is simply a <something>_url property that represents a conforming fully-qualified RFC 6570 URI to another resource. These are intended to ensure that clients do not have to construct their own URIs for related resources, so by coding to use hypermedia links instead of constructing URIs, the future compatibility of the client is improved.
