The principle is that of riding a motorcycle on a windless day, we feel the wind in our faces because of our own movement.
Earth passes through the still ether, and therefore feels the ether wind.
Michelson's interferometer was very ingenious, and sought to compare the phase-change relationship between two halves of a split beam, when they were reunited. It was expected that as the interferometer table was rotated vis a vis the ether wind direction, the transit time over the separate paths of the two beams would alter relative to one another, and this would cause an interference fringes pattern to alter. But the fringes pattern remained stable. This was interpreted as no ether wind and therefore no ether. No one spotted this error, not even Einstein.