The Supreme Mugwump is the name for the head of the International Confederation of Wizards.
The International Confederation of Wizards is a wizarding intergovernmental organisation, roughly equivalent to the United Nations.
Sometime before 1991, Albus Dumbledore was appointed as a representative of the British Ministry of Magic to the International Confederation and became Supreme Mugwump, but was dismissed from his position following the Ministry's denunciation of Voldemort's return in 1995. He was restored as a member after the Ministry's acceptance of Voldemort's return, in 1996, but not as Supreme Mugwump.
If you are asking what does the word mean I have found this.
Mugwump (pronounced /ˈmʌɡwʌmp/) This archetypal American word
derives from the Algonquian dialect of Native Americans in
Massachusetts. In their language, it meant “war leader”. The Puritan
missionary John Eliot used it in his translation of the Bible into
their language in 1663 to convey the English words duke, officer and
captain. Mugwump was brought into English in the early nineteenth
century as a humorous term for a boss, bigwig, grand panjandrum, or
other person in authority, although often one of a minor and