Steady Aim is a gear perk that reduces weapon sway. A steadier sight picture makes long shots and headshots easier, especially with scoped rifles.
Your crosshair drifts less while aiming. You land shots on distant or moving targets more reliably.
Wear one of the listed gloves, wrap or hat. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Sniper's Pocket and Thrifty Shooter make a good long-range set with it.