It's like watching an intergalactic alien battle where the combatants are, for some reason, bound to each other by tendrils of glowing white light. In reality, it's a visualisation of Torchlight II's development, provided by its creators, Runic Games.
What you're seeing are additions, removals and changes made to Torchlight II art, source code and other relevant files over the course of its production. These files are stored in what's called a "source control system". Source control allows multiple developers to work on software (like a game) without overwriting each other's work. It also provides a history of revisions made to each file.
Because this history is available, it's possible to do all sorts of crazy things... like this visualisation. While it's difficult to actually make sense of the specific actions taking place, it's one hell of a show.