It's a feature of Boot, delivered in part by the
-c --checkouts SYM:VER Add checkout dependency (eg. -c foo/bar:1.2.3).
It's for when you have two projects, a library and an application, and the application depends on the library. It lets you develop on both at the same time.
You can try it out yourself with the example repo I show in the video: https://github.com/tmtwd/2modules