at the name of Jesus every knee should bow

Here "knee" is a synecdoche for the whole person. Bowing, or kneeling, is a symbolic act of worship. Alternate translation: "at the name of Jesus everyone will bow to worship him" (See: rc://en/ta/man/jit/figs-synecdoche and rc://en/ta/man/jit/translate-symaction))

under the earth

Possible meanings are 1) the place where people go when they die or 2) the place where demons dwell.