The normal Movement rate of a model is defined by its Movement (M) characteristic value.
During their Movement phase, units can move up to their Movement rate in inches. For example, Men have a Movement characteristic of 4 and so many move up to 4". Elves, naturally faster and nimbler of foot, move up to 5". You do not have to move the full distance allowed, or even at all if you prefer, but you cannot move further than your normal Move rate unless charging, marching, pursuing or fleeing (all of these types of move are discussed later).
Troops riding mounts with barding move more slowly because the weight of their gear impededs movement. A cavalry mount with barding suffers a -1" move penalty.
For example a Knight riding a horse can normally move 8". If the horse is wearing barding then the Knight moves 8"-1" (for the horse's barding) which equals a move of 7".