Heirloom Gear was bought using several different currencies (typically only attainable by max-level characters). It was Bind-To-Account, allowing you to send it to any character on the same server (since cross-server mail is not in the game), regardless of faction.
The gear scaled with level. There are armor and weapons. The armor has modifiers that increase experience gained by the wearer that stacks with other pieces of heirloom gear.
It is a perk that allows you to level up a character faster. They are also highly prized for low level PVP because of how powerful the gear can be at that level.