A good API should be self-explanatory by giving metadata about other resources that could be interesting to explore.
E.g. If you get an invoice, it would be nice to get guidance on where to look for related resources like information about the recipient, where to register payments etc.
We don’t want to clutter up the data in the JSON, so we’ve made a convention about collecting all this extra information under the
_links property is a dynamic part on the object, that could contain information, links to related domains and actions you’re allowed to execute.