Battle command is when you instruct your teammates to kill an enemy. Simply press and hold and select an enemy, and one of your teammates with attempt to take him out. Once the enemy is killed, it will count as an attack command kill. Remember, you are only able to mark 1 enemy at a time as well.
Note: It is suggested that you leave this after getting most of the gun specific kill achievements. You are most likely to get a lot of kills using Attack Command during your "FUBAR" difficulty playthrough.