Hit points and Hit Dice. I conflated them through a misreading of the rule. I couldn't understand why they'd print that a Hippogriff had 3+3 when they could have just told me that it had 6 hit points. I figured it out fairly quickly, but the players had a pretty easy time for a bit.I didn't understand how attacking and negative AC worked. I couldn't put the pieces of the puzzle together. Luckily I met a guy who knew how to play D&D.
What term or rule really confused you, when you first started playing?
First class armor is better than second or third class armor; thus armor class 1 is better than armor class 2 or 3.When I first encountered AD&D 22 years ago, I couldn't wrap my head around how you make attacks rolls.
I still don't know why inverted armor class even exists.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.