The Interaction System is Valve's provided starting point for VR Player functionalities; it contains several features used in The Lab. It is included with the Unity SteamVR Plugin. Moon Motion indirectly contains it, as a result of having the Unity SteamVR Plugin as a subplugin.
The Interaction System provides the following important features: the base Player prefab (which Moon Motion has extended), Teleportation, Interactable, Throwable, and Item Package objects (such as the Longbow), and other core functionalities related to interactivity such as velocity estimation when throwing objects. If you want to become better acquainted with the Interaction System, you can watch Fuseman's video introduction and refer to Valve's included documentation.
You can try out Moon Motion's tweaked versions of the interactable prefabs, and the Player prefab can now use Teleportation alongside other Moon Motion locomotions.
To view all the changes made by Moon Motion to the Unity SteamVR Plugin as a whole, you can read the Custom Changes.
To better understand the relationship between Moon Motion and the Unity SteamVR Plugin, refer to the starting docs.