The Michelson-Morley experiment tried to prove the effect of aether, which was supposed to surround the earth, on the propagation of light.
A single ray of light was launched and using a mirror was split into two, with each one following a different path of equal length. At the end the rays joined up again, and the expected result was the typical interference of rays which had travelled at different speeds; but this was not found.
In spite of all their efforts, the rays which got to the end of the distance, by whatever route they travelled, always arrived at the same time. It was as if light had an unalterable speed, independently of whether it moved in the direction of the aether or in the opposite direction to the hypothetical aether.
You can simulate the experiment by carrying out the activities proposed in the following visual.