What do HD, DC and d% stand for? Thx in advance.

I think it is D&D specific lore.

the small letter d followed by a number (without any space) is used by gamers to indicate a dice.

Thus d4 would mean a four sided die, d6 would mean a 6 sided (common) die and d12 would mean a 12 sided die.

d% means percentile dice. Because an actual hundred sided die would be very unpractical you usually roll a d% by using two differently coloured or marked 10 sided dice. One will be the 1's (0-9) and the other will be the 10's (00-90). The two numbers are then added. If 0 00 is rolled it means 100.

HD is a D&D term meaning Hit Dice (the amount of dice used to roll a creature's hit points). Hit dice can roughly be translated into levels.

DC is a D&D 3rd edition term meaning Difficulty Check. Nearly all actions requiring dice rolls (outside direct meelee) has such a check. If the result of a d20 (20 sided die) +/- all relevant modifiers is equal to or above the DC the action has usually succeeded.