Electronic Arts’ original Battlelog app was a window into the stats of your progression in Battlefield 3. Initially, it didn’t allow any interaction with the two supported games, Battlefield 3 and Medal of Honor: Warfighter, and was a light social app offering limited interaction with friends.
The new Battlelog designed for Battlefield 4 adds a lot more functionality for mobile devices to act as second screen support. The second screen can be used for things like a mini-map to set attack points. You can use the Battlelog app or a browser to change your loadout. An interesting feature is the ability to search for servers through Battlelog without exiting your existing game.
Take a look at the video to see all the features explained in more detail.