Miss Chance comes in a variety of different forms, the most common types would be Concealment, which is a representation of obscurement that causes attacks to miss, with the second most common being that of Fortification, which is a type of Miss Chance that specifically reduces the effectiveness of Critical Hits and Precision damage against you.
Concealment
Over the course of combat, you may have Concealment due to various effects against attacks. Concealment provides you with a flat percentage chance for an attack to miss. This percentage chance is determined by the type of Concealment provided.
To determine whether you have Concealment from a ranged attack, choose a corner of the attacker’s square. If any line from this corner to any corner of your square passes through a square or border that provides Concealment, you have Concealment.
When you make a melee attack against an adjacent target, the target has Concealment if their space is entirely within an effect that grants Concealment. When making a melee attack against a target that isn’t adjacent to you, use the rules for determining Concealment from ranged attacks.
In addition, some magical effects provide Concealment against all attacks, regardless of whether any intervening Concealment exists.
Partial Concealment Miss Chance
Partial Concealment gives you a 20 percent chance that any attacks made against you miss. Your attacker makes their attack normally—if their attack hits, you must roll a d100, and if the roll is equal to or lower than your Miss Chance you avoid being struck. Multiple Concealment conditions do not stack.
Concealment and Stealth Checks
You may make a Stealth check at any point within Concealement while you benefit from Concealment.
Total Concealment
If your attacker has Line of Effect to you, but not Line of Sight, you are considered to have Total Concealment from your attacker. You cannot target another creature that has Total Concealment, though you can attack into a square that you think your target occupies. A successful attack into a square occupied by a target with Total Concealment has a 50 percent Miss Chance. You cannot take Attacks of Opportunities against targets with Total Concealment, even if you know what square or squares your target occupies.
Ignoring Concealment
Concealment isn’t always effective. An area of dim lighting or darkness doesn’t provide any Concealment against a creature with Darkvision. If you have Low-Light Vision, you can see clearly for a greater distance than other creature with the same light source.
Varying Degrees of Concealment
Certain situations may provide more or less than typical Concealment, and modify the Miss Chance accordingly.
Fortification
Fortification is the ability to shrug off devastating attacks that target the weak points of your body. Whenever a creature with Fortification would be subject to an attack that deals Precision damage, or an attack that critically hits and confirms, roll a d100 and compare it to your Fortification score, if the d100 is equal to or less than your Fortification score, the critical hit or Precision damage is negated entirely as if it never happened.