Well, I thing the phlosephy went something like this:
ac 10, normal human target with no armor. as armor type is more protective, so o shall the number increase. AC 0 is considered unattainable by any armor with out dex andor magic.
THAC0 was the number with all of the plusses on an attact that was needed to hit this particular number. It is the same as hitting an AC of 20 basicly for 3.x.
If a fighter had a strength of 17 he/she had a +1 to hit and +1 to damage. If the weapon used had a +2 enchantment there was now a total of +3 on the attack.
If said fighter waws level, say 5 then the thacl would be a base of 16, then add previous attack bonuses and you now have a THAC0 13.
Now if you were up against an otherworldly creature with an ac of -5 such as a typ of deamon, then you would now need an 18 to hit.