Do you also want us to explain THACO?
In the second edition rule set for AD&D the lower your Armor Class is the better. THAC0, is an acronym that stands for: "To-Hit-Armor-Class-0". You must 'roll' your THACO or higher, on a twenty-sided die, in order to get a hit on an enemy with an AC 0. If your enemy's AC is higher than 0, the difference is added to your roll, if it's lower than 0, the difference is subtracted from your roll.
For instance, say your THACO is 10 and your are battling an enemy with an AC 5. You must roll a 10, or higher, to hit AC 0, but since his AC is 5, you can roll a 5 and still get a hit. The difference of 5 would be added to your roll of 5, thus reaching your THACO of 10. So, in short, the lower your THACO, the better for you.
As your character progresses in levels their THACO will decrease thus increasing their chances of hitting a lower (better) armor class. In game terms this is basically your character getting better at fighting.
An example: Lothar "the generic fighter" has an AC of -2. and Hzzuk "the evil troll" has a THACO of 12, and a +2 to hit from his strength.
This would calculate out as follows: For Dave the Troll to hit Jay he would have to roll a 12 or better. (THACO of 12 to hit AC0, Jay has an AC-2. Normally, Dave would need a THACO of 14 to hit AC-2 but he has a strength bonus that grants him a +2 bonus to hit so the roll required is reduced by 2 down to 12. Simple right?
SOURCE:
http://www.bioware.com/games/shadows.../faq/?faqID=41