With Nohr, the dark kingdom, you must fight with the royal family of Nohr to change the reign. You will have plenty of flexibility and liberty out of the story, being more recommended for those that preferred the liberty Awakening gave. With Hoshido, the light kingdom, you must protect your homeland, from the crushing Nohr invasion. There's even more content you will have to pay for, such as maps, a character that expands the story of the route and add adds even more content with another route. Though the most important decision is to pick a game to buy, since it's split in two (Hoshido or Nohr), being those the paths you will have to choose, having to spend more money if you want to see everything as DLCs or expansions, though at the same time everything is announced as a single game.

'Fates' is something important, since the decisions will affect history, to the point it will change the path you will go through. You, as the protagonist, descendant of dragons, is from the royal family from the peaceful kingdom Hoshido, but raised by the royalty of the kingdom seeking for glory, Nohr, who are also descendants of dragons. There being a peaceful and calm kingdom, and another that seeks more, the relationships between both countries reach their limit and a struggle is about to start, and you will have to choose a side. A battle you must pick, a future you must choose.

Fire emblem fates chapter 26