Hello
@qperteplex has the idea but some things should be clarified
He is correct about the critical rate (the number you see on the stats window). that stat just shows your chance to do a critical hit (an attack that causes about 40% more damage than normal)
In your picture, you should have 54% chances to do a critical hit each time you attack someone
according to some sources, the stat that the windows shows is not the real percentage and its due to how game calculates such stats
the formula to show stat in window is: (LUK / 3 ) + bonus = stat %
Easy Example: (30 LUK / 3) + 10 bonus = 10 + 10 bonus = 20 critical % stat
the formula for REAL Critical Rate is (LUK * 0.3) + bonus = real%
example with same stats: (30 LUK * 0.3) + 10 bonus = 9 + 10 bonus = 19 real critical rate
for this reasons, most guides recomend to get 100+ on crit [stat], making the REAL crit 100%
Lastly... the critical shield that he and niemono89 talk about is the perfect dodge that every enemy has (the chance to completly evade any attack)