Instead of extracting part of The Path and releasing it as a free trial, this is a brand new chapter, similar to parts of the game, but not the same. It gives you a free taste of the atmosphere of The Path.
In The Path - Prologue, you control the mysterious Girl in White. Theres a not a lot of gameplay in this demo. Its just about exploring the forest and pondering the sights. A prologue should show what happened before the events in the story. But in this case, it may seem like its showing things that happened after. A mystery that will make more sense after playing the full game.