From DDO Compendium
OverviewCritical hits and sneak attacks produce higher damage and can be deadly. Fortification represents your chance of downgrading these types of attacks to normal attacks instead. Fortification only helps you against critical hits and sneak attacks.
EffectWhen you are hit by an attack that is a critical hit, or one that deals sneak attack damage, you have a chance to reduce the damage you take from that attack equal to your fortification - the target's fortification bypass.