WoW Legion Companion is the official companion app for the latest World of Warcraft expansion, Legion. It's designed to let you follow your character's progress, checking his or her stats and open quests at any time, or even seeing where he or she is on the map.
Obviously to use WoW Legion Companion you also need to play World of Warcraft: Legion, have a Battle.net account, and have a WoW subscription. Only if you can meet all three of these prerequisites will you be able to use the app.
You can use WoW Legion Companion for loads of different things, including checking what missions are available at the class halls. In addition, you can get rewards by completing missions, as well as manage followers, improve your equipment, or recruit troops for future missions.
WoW Legion Companion is a very useful companion app. With it World of Warcraft: Legion players can really get the most out of the game.