I have no idea how the basic D&D stats actually function in this game, so I mostly consider them flavor. Sure characters with higher Con have more Hitpoints, but there is no way of changing their Con, so it doesn’t really matter.
The stats you really care about here are Attack, Damage, Hit Points, and Armor Class. Two of these are very easy to understand, Hit Points is how much damage you can take, and Damage is how much damage you deal (or sometimes heal). Attack and AC are slightly more complicated, and are actually linked.
First of all, neither affect your chance to hit or crit. Your hit chance is 95% and your crit chance is 5%. There are some effects on gear that modify these numbers, but stats do not. What these stats do is modify the damage dealt.
You’ve probably noticed that a character will sometimes hit outside their stated damage range, that’s a result of the attack vs AC calculation. The game divides the attacker’s attack value by the defender’s AC value. The result is multiplied by the attacker’s damage range to determine the damage range used. Then a number in that new range is randomly generated and becomes the damage dealt.
There are caps on this calculation though. If the Attack/AC ratio is higher than 3, it is set at 3, so you will never hit for more than triple your stated damage range (non crit). And if the Attack/AC ratio is less than 1/2 it is set to 1/2, so you will never do less than half damage. That’s why Tommus can still die even if he gets a billion AC.