The D is short for Dungeon, so it means Dungeon 3 which is the current final dungeon in the latest version. You should prepare for your HP throughout the leveling process since it is recommended to have a lot of HP anyways.
So like Vestra said, put all your points into your main stat (agi for ranger, then respec to int when you become druid). Then put the rest of your points into HP. When you respec to a Druid, however, put about 1-3 points into Wisdom, then all of your points into int and the rest into HP.