In old-fashioned printing, a kern is the part of metal type that extends beyond the letter's block of type, like the tail of an f or a j.
The verb kern comes from the noun. When you move a metal piece of type closer to or farther from its neighbor, you kern. This aesthetic adjustment between the printed letters of a word is very important to designers and typographers, and it's known as kerning. Kern comes from the French carne, "projecting angle," from the Latin root cardinem, or "hinge."