This is a special perk, meaning that it is not gained simply by picking it upon leveling up. It is an implant named Reflex Booster. It is available from Doctor Usanagi at the New Vegas medical clinic for 4000 caps. It is designed to boost the reflex action of the recipient by conducting the electric current in the nervous system and sending it to the area where it needs to go to, therefore increasing the recipient's Agility by 1.
If the character's Agility stat is 3 or less, Dr. Usanagi will offer a discount of 1000 caps on this implant.