When you want to activate a unit, you need to decide whether it is going to move (without fighting), attack (move into combat and fight immediately), or shoot (which only applies to troops with missile weapons of course).
Each time you want a unit to carry out any of those actions, you roll two six-sided dice and check your total against that unit’s ability; some units are better at certain commands than others. For example, Archers need to score a 6 or more to shoot, which represents their faster rate of fire than Crossbowmen who need a 7 or more. Mounted Men-at-Arms like to get stuck into their enemies and do so on a 5 or more, but they’re less willing to trot around the battlefield guarding sheep or defending strategic points (a non-combat move requires a 7 or more).
This means there’s a wide variety of effectiveness between different units for different actions, so it is best to field a Retinue well-suited to the scenario you’re playing, or a Retinue that can cope well in all sorts of situations.