It was previously only available on Windows Phone, but now 343 Industries have made iOS and Android friendly versions of ATLAS.
The App includes: Weapon and vehicle spawn locations for Matchmaking and Firefight games; health pack locations; near-real-time locations for weapons and vehicles during Custom Multiplayer games; your location and that of your team members displayed on the map; ‘follow me' mode to track your movement; dynamic player list, including current score for the teams and each player and equipped weapon for you and your teammates.
If you're serious about your Halo then this is probably an essential download. For more details have a look on Halo Waypoint.