Troops who are behind certain terrain features, such as hedges or walls, or inside a wood, can take advantage of cover. This makes them harder to hit because they can duck back out of the way, leaving arrows to splinter against a wall or tree. A unit is considered to be in cover if at least half of its models are in cover (or half of its body in the case of units consisting of single models).
If you find this hard to judge, you can just roll for it (see The Most Important Rule!).
There are two sorts of cover: hard cover and soft cover. Their effects are explained in the To Hit Modifiers chart.