In the 19th century, physicists generally believed that just as water waves must have a medium to move across (water), and audible sound waves require a medium to move through (air), so also light waves require a medium, which was called the "luminiferous” (i.e. light-bearing) “ether”.
The Michelson-Morley experiment became what might be regarded as the most famous failed experiment to date and is generally considered to be the first strong evidence against the existence of the luminiferous ether. Michelson was awarded the Nobel Prize in 1907, becoming the first American to win the Nobel Prize in Physics.
Physicists had calculated that, as the Earth moved in its orbit around the sun,
the flow of the ether across the Earth’s surface could produce a detectable "ether wind". Unless for some reason the ether were always stationary with respect to the Earth, the speed of a beam of light emitted from a source on Earth would depend on the magnitude of the ether wind and on the direction of the beam with respect to it. The idea of the experiment was to measure the speed of light in different directions in order to measure the speed of the ether relative to Earth, thus establishing its existence.