Font has two meanings: "a style of typeface" and "a basin of baptismal water in a church." Add a "u" and you get fount, which is a source of something important like wisdom or inspiration.
The word font is most often used to mean the specific size and design of printed type or computer text. Its Latin root is fundere, "to melt or cast," referring to the way 15th-century fonts were made, by casting molten metal into moveable type that was used to print on paper:
The paper was thinner than in the other books, and the font was tiny. (Amal Unbound)
An earlier meaning of font, "a receptacle that holds holy water," is from Latin fons, "fountain." In some Christian churches, baptisms involve a clergy member sprinkling or pouring water from a font:
I propped Raulito up on the baptismal font and gave the other children mints to keep them quiet. (In the Time of the Butterflies)
Fount shares a root with the religious font and was probably shortened from fountain. It's been used figuratively since the 16th century to mean "a flowing source:"
By then, he was seen as an elder of the game, a fount of timeless wisdom who had a comment for every occasion in life, including death. (The Washington Post)
He had a simpler nature; he looked up to Balthamos as to the fount of all knowledge and joy. (The Amber Spyglass)
In common usage, font can also mean "source," and many style guides and English teachers consider both words to be correct. However, sticklers who consider themselves founts of grammar expertise tend to prefer fount. Hence the common expression "fount of knowledge."