Characters can have hit dice up to their level, so 2 hit dice for a level 2 character. But long rests can only restore 1/2 of your maximum hit dice at a time. So, to regain 2 hit dice at level 2, you have to long rest twice. For the non-D&D folks, hit dice are like health potions, and can restore a certain amount of health when you short rest. More hit dice = more health restored #dev