Taking cover behind a wall, tree, or other obstacle provides a +4 bonus to Defense. Cover is measured relative to the attacker. For example, hiding behind a low wall provides no cover against an opponent hovering above you, but does provide cover against an opponent on the other side of the wall.
Cover and Reflex Saves
Cover grants you a +2 bonus on Reflex saves against attacks originating from a point on the other side of the cover from you.
Cover and Stealth Checks
If you cannot draw a line between you and a target without intersecting cover, the target has total cover. You can't make an attack against a target with total cover.
Varying Degrees of Cover
In some cases, cover may provide a greater bonus to Defense and Reflex saves. In such situations, the normal cover bonuses to Defense and Reflex saves can be doubled (to +8 and +4, respectively). A creature with this improved cover gains a +10 bonus on Stealth checks.
If it ever becomes important to know whether the cover was actually hit by an incoming attack, the Narrator should determine if the attack roll would have hit the protected target without the cover. If the attack roll falls within a range low enough to miss the target with cover but high enough to hit the target if there had been no cover, the cover is hit. This can be particularly important to know in cases when a character uses another character as cover.
In such a case, if the cover is struck and the attack roll exceeds the Defense of the covering character, the covering character takes the damage intended for the target. If the attack roll is lower than the Defense of the covering character, but higher than the Defense of the covered character, the original target is hit instead. The covering character avoided the attack and didn't provide cover after all! Covering characters can voluntarily lower defense bonus to ensure they provide cover.