like all the nobles in the country got together in a big chamber and voted on stuff. The king was very very weak. Eventually just one noble could veto anything so literally nothing got done after a certain point. Poor people were kept out and serfs were still treated like shit but since the requirements to be considered a noble were so loose a lot of middle class merchant types snuck in too. Poland would later be one of the hotbeds of republican revolution during the Napoleonic wars.