The video below describes the basic understanding that magma upwells in the mid-Atlantic ridge to form new rock as the Atlantic Ocean slowly widens. But the rock at the ridge has its iron ions locked in oriented to our current magnetic orientation, yet as you look side to side there are many stripes of rock with reversed magnetic orientation. The easy explanation is that the magma is always slowly oozing out and solidifying in the magnetic field of the Earth, but that the Earth's magnetic field periodically reverses.