Cover is considered as any obstacle between you and your target. Other people, barrels to duck behind, trees, and walls are all considered cover. Cover will modify the to hit chance as a negative penalty. You will never have a better chance to hit your target if there is cover between you and it. The amount of the penalty depends on the cover. A barrel may be very little, another person a little more, and a wall may have a really big modifier. Targeted attacks are discussed below, but they all have some sort of negative to hit penalty based on the location being targeted. Trying to hit someone in the eyes is more difficult than attacking their left arm (or tentacle, or whatever).