What is it?
In short, "Photoshop Layers Workflow with Scenes". Curious? Check out the documentation.
When Unity was young, projects were small and organization was simple: each scene was a different level and that was that. As the new, fancy lighting features have demanded special handling, partitioning levels into static and dynamic objects has been necessary. As teams went from one or two developers to dozens, splitting scenes has become a lifesaver to keep everyone from stepping on each other's changes.
Additive loading scenes is supported in Unity, but there isn't a good way to really use it effectively. For example, the lighting artist might want to place lights in a scene separately from the artist building architecture. The lighting artist doesn't want to accidentally save the architecture scene, even if something got changed. In vanilla Unity, if you hit save, all the modified scenes save and now there's a merge conflict in the architecture scene. Oops.
With Split/Scenes, you can lock the scenes you don't want to save, and only unlock the ones you're working on. It also shows you which scenes are modified, and if you lock a modified scene, it warns you that you will throw away information. Split/Scenes is made to make your life easier, give you better control, and prevent the mistakes that are generally hard to prevent through diligence alone.
So I created Split/Scenes to make that easier. I think it's a great tool, and I hope you do too. I'm open to suggestions for improvements, too!