How to get heirloom shards?

So, I'm curious about how one goes about acquiring heirloom shards in this game. Is there a specific quest or challenge that needs to be completed? Are they randomly dropped from certain enemies or chests? Do I need to have a certain level or rank to even have a chance of getting them? And once I have some, what can I do with them? I'm really eager to find out more about this aspect of the game and how I can incorporate heirloom shards into my gameplay strategy.