Rulers can give their vassals roles in the governance of the Kingdom, you can see what roles a player has by looking at the icons to the left of their username. Roles give some benefits to the player, but they also show other players that the player has the confidence of the king. Each role can only be given to one player at a time, players can hold multiple roles, but only the most recently assigned role will be shown.
List of roles:
Heir: will become king if the ruler abdicates or become inactive also can manage diplomacy on behalf of their king.
Chancellor: a Lord who helps negotiate diplomatic treaty's with other kingdoms.
Marshal: a Lord who co-ordinates the attacks and troop movements of the other Lords.
Archbishop: helps new players to build up their settlements, distributes resources etc.
spymaster: watches out for signs of rebellion.