An attack bonus item property (on a weapon) makes a character's attacks (with that weapon) more accurate by adding an amount to a character's attack bonus. This also allows those attacks to pierce damage reduction; a weapon with a +2 attack bonus property ignores damage reduction of X/+2 and lower.
Multiple attack bonuses on an item do not stack with each other, nor with the attack aspect of an enhancement bonus. For example, a weapon with a +3 attack bonus and a +5 attack bonus versus dragons will add +3 to attack rolls against non-dragons and +5 to attack rolls against dragons. Similarly, a weapon with a +5 attack bonus and a +3 attack bonus versus dragons will add +5 to all attack rolls, regardless of what is being attacked.
The bonus from this item property is subject to the +20 attack bonus cap.