Hypermedia places heightened operational significance on resources, located at URIs. With a hypermedia API, when a request is sent to a URI, the response lists information on resource state and acceptable future operations, in essence creating a state machine that can be manipulated.
Hypermedia is truly valuable as it can create more powerful APIs that alleviate the need for API versioning.
HATEOAS is a constraint of the REST application architecture that distinguishes it from most other network application architectures. The principle is that a client interacts with a network application entirely through hypermedia provided dynamically by application servers. A REST client needs no prior knowledge about how to interact with any particular application or server beyond a generic understanding of hypermedia. By contrast, in some service-oriented architectures (SOA), clients and servers interact through a fixed interface shared through documentation or an interface description language (IDL).
The HATEOAS constraint decouples client and server in a way that allows the server functionality to evolve independently.