Cover (EU2012)

In combat, soldiers and aliens can take cover behind objects in the environment to lower their chance of being hit by enemy fire. Cover is taken automatically as the soldier moves through the environment. While in cover the soldier gains a defensive bonus against enemy attacks unless the enemy has flanked the soldier. Cover is divided into two categories: Low Cover and High Cover.

Low Cover
Low Cover is represented by a half shield symbol on the tactical HUD. It conveys a -20% accuracy for the enemy firing at the target.

High Cover
High Cover is represented by a full shield symbol on the tactical HUD. It conveys a -40% accuracy for the enemy firing at the target.

At Classic and Impossible difficulties, it's better to try to stay out of line-of-sight of aliens whenever possible (assuming you are not going on Overwatch yourself). When overwatching be aware of what accuracy percentage you are likely to get. It is better to hunker down and double your defense bonus rather than risking a 20% shot. Especially early on with rookies with low accuracy and carrying weak weapons it isn't worth the risk.