Hit dice represent the amount of
hit points, measured in
d6 (six-sided dice) that a
character,
monster or
NPC has.
Usually used in the AD&D RPG, it is commonly used for determining the relative health of a newly-encountered monster. When first spotted, the Dungeon Master rolls the appropriate number of d6s to see how many HP the being has and then allows the characters to interact - steal, stab or spell - with it...