This is a Unity Package Manager compliant package repository for the External Dependency Manager for Unity (EDM4U) (formerly Play Services Resolver / Jar Resolver) developed by Google, officially located at GitHub. It is an alternate distribution of the libraries, intended for formal Unity packages and not informal unitypackage files as the distribution method.
To install this package into your Unity project, you must add the following scoped registry to the Package Manager Settings:
Name: Chartboost URL: https://registry.npmjs.org Scope(s): com.chartboost
This will allow the Package Manager to show Chartboost's available packages using the "My Registries" filter.
See the documentation on GitHub for the background.