What Do These Numbers Mean
There are other numbers involved in AD&D, but these are the most common. If you have a specific question, Email me!
What are all those numbers for?
As stated earlier, when you play AD&D, you say what your character does in a given situation. More accurately, you say what your character tries to do... your DM may rule that you're simply not able to climb a sheer 90 foot rock wall, or slay a giant with one blow. Much of this is controlled by your character's statistics... how strong your character is, how quick, how tough, etc. How does a DM determine all of this? While a great deal of both imagination and preperation are required for a good game, there are some rules for common occurences... i.e., whether your sword hits that giant or not.
You may have seen the strange-looking dice used by AD&D players. These are used, in conjunction with your character's stats, to determine how well you do many things.
AC (Armor Class): This number shows how difficult you are to hit; the lower, the better. Quick, nimble characters are harder to hit, as are characters wearing heavy armor. Usually, 10 is the worst, and -10 the best.
THAC0 (To Hit Armor Class Zero): Your skill in combat. The lower, the better. You must roll this number or better on a twenty-sided die to score a hit in combat. Since most things don't have an AC of 0, you subtract their AC from your THAC0 determine what your roll needs to be.
HP (Hit Points): How much damage you can take before being knocked unconsious/killed. The higher, the better.
Die what? Oftentimes, you'll hear someone say something like 'die eight' or read: 'd8'. What both of these mean is 'eight-sided die'. So, if what you were attempting to do during a game required you to roll 2d6, you would roll two dice with six sides each. 1d20 is one die with twenty sides. 3d4 is three dice with four sides, and so on. More important stats: Characters in AD&D are also defined by six major attributes that usually range from 3-18 (with about 9-13 considered to be an 'average' person)
STR (Strength): How much you can carry, how hard you can hit. High strength improves your chance to hit in hand-to-hand combat, as well as allows you to carry and break more things.
DEX (Dexterity): How agile and precise you are. High dexterity improves your armor class and increases your chance to hit with ranged weapons like bows.
CON (Constitution): How tough you are. High constitution gives you bonus hit points and gives you increased resistance to poison and the like.
INT (Intelligence): How smart you are. Intelligence is a must for magic users, and determines which spells can be learned.
WIS (Wisdom): Intuition, how well you understand the world. Important to priests, druids, and rangers.
CHA (Charisma): More abstact than the others, this combines looks, communication, and sheer force of personality. High charisma helps you to get your way and lead others.
What if I get in a really sticky situation? This is where a number called a 'saving throw' comes in. Saving throws are rolled in situations such as: a dragon breathes fire at you, and you have a split second to leap out of the way. You fall from a long way... did you tuck and roll, or fall flat? A wizard casts a sleep spell on you... can you shake it off, or will you start snoring? As you gain in experience, you get better at saving throws
Thanks to t_catt11 for this contribution!