Armor and devices are available in different qualities - Uncommon, Rare, and Epic. While higher-quality pieces have more modifiers, they also have a durability stat, ranging between 0-100. Every time the piece is used in a mission, the durability is decremented - when it reaches 0, the item acts as if it were a common item, losing all of it's bonus modifiers. Note that as the armor or device decays, the upgraded stats gradually decline (ie. a healing mod of 6% will reduce to 4% then 2% and so on).
Repait Kits are craftable items that restore Armor and Devices to full durability, restoring the bonus modifiers. Repair Kits can only be used on items of the same quality - an Uncommon Repair Kit can only restore the durability of Uncommon Armor and Devices.