Syndicating in Warband means corrupting groups of nobles for your interest. This can have many benefits, such as messing up the kingdom's unity, convincing lords to join your side and turn against their king, or forcing someone to be rejected and exiled from the kingdom, meaning that he joins another. But it can be problematic if the lords who joined you have rivalries, and you must do your best to prevent it and keep your subjects loyal to you.