What is "Tommy John surgery"?

Which player won the most World Series rings as a player, with 10? Who was baseball's first commissioner, appointed after the Black Sox scandal? Who has served as MLB commissioner since 2015? Which commissioner led MLB from 1992 to 2015? Which surgeon first performed Tommy John surgery in 1974? What is the "Mendoza Line" in baseball? What is the "infield fly rule" designed to prevent? What is a "balk"?