Tactical Pocket is a gear perk that raises your SMG ammo reserve. SMGs fire fast and empty magazines quickly, so this helps keep an SMG build going through long fights.
You carry more SMG rounds, so you can keep spraying in close fights before you need to resupply.
Wear the Breach & Clear Pants in the legs slot. 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.
Worth it if an SMG is your main gun. If not, pick a defense or noise perk for the legs slot.