Here’s the basic rundown for anyone else that’s curious.
A service is a method or set of methods inside of a package that another package can use to interact with the providing package. It essentially allows the developer to expose a public API to their package. It also defines the dependency relationship between two packages. A package cannot depend on another package outright, but it can depend on the services that another package provides. (I’m assuming this in essence means that if the user installs a package that consumes a service of a second package, then that second package would be installed as well. Someone please correct me if I’m wrong.)
As an example, let’s say that a user wanted to create an Auth package. The Auth package would list its versioned services through the package.json file. Another package could then specify through its package.json that it wanted to consume one of the Auth packages services.
That’s at least what I gleamed from the listed issue.