Focused Shot lets you trigger Focus, a short slow-motion window, while aiming a ranged weapon. It is a Tier 1 Ranged skill and most players buy it first, because many later skills build on Focus.
Yes, buy it first. It makes headshots easy and gets you out of bad fights with Breakers, Ragers and Hordes.
Crowd Control and Under Pressure. Shorter cooldowns and free reloads make Focus last a whole fight.
Save Focus for dangerous targets such as Screamers, Breakers and Marauders on rooftops. Burning it on Swarmers wastes the charge.