[Quiet Man Cometh]

THAC0 is particular to ADnD and means "to hit armour class 0" and that opens up a whole new pile of explanation to define. It was a round-about way of calculating whether or not an attack hit, and was gotten rid of for later editions. It's affected by level, class, and weapons as well. The higher your level, the better you are, the better the weapons, the better you are. Mage THAC0 sucks, Fighter THAC0 good. THAC0 of course only applies really to physical attacks so a mage having sucky THAC0 doesn't matter as much unless they go after a bad guy with a dagger or something.