Hit points is how much damage a unit can take, a 7 hp infantry can take 7 hits, so if a tank attacks it and rolls higher then 7 it kills it, critical chance is the chance of a critical hit being scored (10 being 10%, 50 being 50%) it adds a guaranteed damage (like a tank has 8 attack, with a critical hit it automatically calculates that attack in its damage output with aditionaly rolling another attack so you can roll 4 and the attack will be 12 all in all).