Concealment Miss Chance
Concealment gives the target of a successful attack a chance that the attacker actually missed. This is called a miss chance. Normally, the miss chance for concealment is 20%. Make the attack normally; if the attacking creature would hit, the target must roll a 20 or lower on a d% roll (see page 513) to avoid being struck. Multiple concealment conditions do not stack.
Source
Core Rulebook pg. 253
