Light Armor Kit

A Light Armor Kit is applied to armor to increase the item's armor value by a value of +1. It can only be attached to chest armor, boots, leg armor, or gloves.

Armor kits fill the same virtual "slot" as enchantments and will overwrite any existing enchantments, or be overwritten by enchantments applied later.

Source
This item can be crafted by Leatherworkers starting out with a skill level of 1.

The pattern to make Light Armor Kit is learned from a Journeyman Leatherworking Trainer.