Thac0 - Let us say your character's Thac0 is 10.
Now, your target's Armor Class is -2.
You roll a 10 on the 1d20, you subtract 2 since the target has a negative armor class, thus giving you an 8 on your roll, you miss.
And, if I am not mistaken, Thac0 is a combination of proficiency, stat...