The Council of Constance issues the decree Haec Sancta Synodus, declaring that a church council is the supreme authority within the Catholic Church — superior even to popes, who are obliged to obey the dictates of councils.
The reason why the council issues Haec Sancta Synodus is because they are dealing with a massive schism in which three rival lines of popes are all claiming legitimacy. The only way the cardinals and bishops see to resolve the issue is by claiming authority over all of the rival popes and starting over.
According to the decree:
"Legitimately assembled in the holy Spirit, constituting a general council and representing the Catholic church militant, it has power immediately from Christ; and that everyone of whatever state or dignity, even papal, is bound to obey it in those matters which pertain to the faith, the eradication of the said schism and the general reform of the said church of God in head and members."
This decree of the Council of Constance is considered the height of the conciliar movement in the Catholic Church. It will never be accepted as valid by the Magisterium of the Catholic Church, however, because no pope ever approves it.