Per the developer documentation
Integration style web API’s enable a single web API call to run multiple services at once for a more efficient integration. An example of this behavior can be see in the Catalog where one web API call can create a product; if your payload includes the inventory object and media object then the framework will also create the product’s inventory & media in that one API call.
Create a new integration on Magento Admin. To create an integration, click System > Integration > Add New Integration. Be sure to restrict which resources the integration can access.
It's not all that clear what, exactly, an integration is. The first documentation snippet indicates an integration is (maybe?) a way to chain multiple API calls in a single HTTP request. However, there's no syntax example of this. Also, if I use the information in the second documentation snippet I can create an integration object, but it's not clear what I'm supposed to do with this. Also, digging into the code, integration objects appear to use a different authentication code path.
Does anyone have a clear idea what these "API Integrations" are, and how they work?