Focus appears as a Resistance perk in Call of Duty: Ghosts. It reduces flinch when shot and reduces idle sway while aiming down sights. It is unlocked normally at level 30 or can be unlocked earlier by purchasing it for six Squad Points. It is unlocked by default with the "Marksman" soldier option.
The amount of flinch reduction received is significant, and the effect Focus has on idle sway is very useful, considering almost every weapon in the game has idle sway.
This perk has a great impact on Sniper Rifles, as those weapons require the most precision and have the most idle sway. By using Focus, the player is less likely to have their aim thrown off during heated gunfights, and helps them maintain accuracy and quite possibly save the player's life.