The Supreme Chancellor was the celebrated individual who
bribed Senators was elected by his fellow peers to sit in the most comfortable seat in the Senate. This meant little increase of Powah, since the Galactic Republic was a democracy, and each and every last single one of Senators had a equally important vote. *Sigh*
After a quadrillion years, Supreme Chancellor Palpatine got the right idea with his new You can keep your seat and your meaningless vote in this obsolete council which does not really exist anymore but dare you to defy me I'll destroy your planet Reformation. Crowning himself Emperor, the silly title of Chancellor was no more.
Behind the scenes
In Gungan, Supreme Chancellor is translated as: Suspreemes Chanslorry Meesta