When you reach level 31 you get the option to retire. This brings your character back to level 1, but you keep all items and gold. It also gives you a new generation (30 more xp per multiplier per generation) and you can choose a single item to heirloom once. This is commonly called a heirloom point.
Note that you don't get any actual points and you can't retire without selecting a item to heirloom. So when people sell the points, they have to get the item to be heirloomed from the buyer, heirloom it, then give it back.
