Before the Fall of Oriath update, Path of Exile had four acts which were then repeated twice with slightly higher difficulty. Acts are what split the areas of the game and the story up, similar to a level in a more linear game or a chapter in a book.
Not only does the update add a fifth act, in which you return to Oriath where your character originally comes from, but the structure afterwards also saw some changes. Five new acts replace the cruel and merciless difficulty versions of acts, and while some of the general areas and important locations are used, there are a lot of new areas that you will have to explore. Rather than playing through the story three times, you’re going from start to finish once.