Calm At Heart is a gear perk that improves your health regeneration while your weapon is holstered. It helps you recover between fights without using a med kit.
Holster your gun and your health comes back faster. It is useful while exploring or sneaking between targets.
Wear one of the gear pieces that carry it: Fry Cook Apron. Each piece only takes one body slot, so plan around the other perks you want.
A small comfort perk. It does nothing during a firefight, so combat perks usually win the slot.