From FM 7-0 Training the Force:
Battle focus is a concept used to derive peacetime training requirements from assigned and anticipated missions. The priority of training in units is to train to standard on the wartime mission. Battle focus guides the planning, preparation, execution, and assessment of each organization's training program to ensure its members train as they are going to fight. Battle focus is critical throughout the entire training process and is used by commanders to allocate resources for training based on wartime and operational mission requirements. Battle focus enables commanders and staffs at all echelons to structure a training program that copes with non-mission related requirements while focusing on mission essential training activities. It is recognition that a unit cannot attain proficiency to standard on every task whether due to time or other resource constraints. However, commanders can achieve a successful training program by consciously focusing on a reduced number of critical tasks that are essential to mission accomplishment.
For more information see: FM 7-0 Training the Force