Troops on a hill can draw a line of sight from an elevated position. To determine what me troops can see, you should get down over the table and take a model's eye view. Therefore, most of the time your troops may be able to see over troops that are on the ground below, and over other obstacles that are lower down the hill.
Troops on a hill are considered to be in a good position to fire, so can fire with one additional rank compared to missile-armed troops on flat ground. This means that most bowmen can shoot with two ranks if they are on a hill.