This is a special perk, which means that you can't just pick it when you level up. It is an implant named Reflex Booster. You can get it from Doctor Usanagi at the New Vegas Medical Clinic for 4000 caps. It is designed to boost a person's reflex action once it's installed. This perk will increase your base Agility by 1.
If your Agility is 3 or less, Dr. Usanagi will offer a discount of 1000 caps on this implant.