Additionally, certain characters will automatically deal a Critical Hit when their Adrenaline Rush is activated.
Crits increase damage by 50% against humans, but note that your crit stat isn't a straight percentage change to crit. There's some more complex math that happens to convert your crit 'rating' into a chance to crit, which is designed to apply some diminishing returns so people don't get into a state where they're critting with every attack (which would totally break all walker combat). Dash *Scopely*
Further information on the use of Critical Hit can be found on Wikipedia.