Fire Emblem Fates is split into three story paths. Birthright is one of the two base stories, the other being Conquest. The other story path can be bought digitally at a discount through the in-game store. The third story path is Revelation, a DLC that was released later. You need to be have completed Chapter 6 in either game to access the other stories and DLC. Birthright is the easier path as it has the ability to use scouting missions to grind experience and money and simpler mission objectives.