Armor Master is a craftable set. While you have an armor ability slotted, your Max Health is increased by 5%. When you use an armor ability, your Physical and Spell Resistance is increased by 5332 for 10 seconds.
You need to have an armor ability slotted for the 5-piece bonus to work (either the light, medium or heavy armor ability), otherwise you will not benefit from it. The armor ability has to be slotted on the bar that you are using when you want the 5% health increase. If you activate the armor ability and you swap to a bar without an armor ability, you will lose the extra health but keep the resistance.
The Armor Master set is part of the Imperial City DLC. The crafting station is located in the Memorial District of Imperial City. You can craft any armor, jewelry or weapon with this set in any motif style you know, as long as you have 9 traits researched. Crafted pieces can be traded to other players. Each set item becomes bound on equip.
If you need more information about the Imperial City, make sure to check out the full Imperial City Guide