Troops take advantage of cover to hide from enemy missile fire. This makes them harder to hit because they can duck back out of the way leaving arrows to splinter against a wall or tree. There are two sorts of cover: hard cover and soft cover.
Hard cover offers real physical protection as well as partially concealing the target from view. The corner of a building, a large rock, walls and wooden pallisades are all hard cover. Troops positioned at windows and doors also count as being in hard cover. A model positioned at the corner of a building so that he is peeking round is behind hard cover. Models in trenches or pits are also counted as behind hard cover.
Soft cover partially shields a target from view but provides little actual protection against incoming missiles. You can hide behind a hedge but a crossbow bolt or an arrow will go straight through it. Hedges, wagons, carts and woodland all provide soft cover. Troops within woods automatically count as being in soft cover.