The Kul Tiran Human Heritage Armor Set is a transmog set that can be used by any Kul Tiran Human, but not by any other race. It's pretty easy to obtain, and all you need to do is to level a Kul Tiran human to level 110.
When you hit level 110 on a Kul Tiran human, you get a quest that you can instantly complete, and it rewards the entire set. This gets automatically added to your Wardrobe, so you can transmog it to any item at this point. When you have unlocked it on one character, all your Kul Tiran humans can transmog it, even those who are below level 110.
It's important to keep in mind that you will not get this transmog set if you use a character boost to get you to level 110, and you will need to manually level up to 110 to get it.