HATEOAS (Hypermedia as the Engine of Application State) is one of the constraints of the REST application architecture.
Let's consider a situation where a service consumer is consuming numerous services from a service provider. The easiest way to develop this kind of system is to have the service consumer store the individual resource URIs of every resource they need from the service provider. However, this would create tight coupling between the service provider and the service consumer. Whenever any of the resource URIs change on the service provider, the service consumer needs to be updated.
Consider a; typical web application. Let's say I navigate to my bank account details page. Almost all banking websites would show links to all the transactions that are possible on my bank account on the screen so that I can easily navigate using the link.
What if we can bring a; similar concept to RESTful services so that a service returns not only the data about the requested resource, but...