The Michelson-Morley experiment was actually a series of experiments carried out between 1881 and 1887. These experiments attempted to measure the Earth's movement through a medium that was then thought to permeate the vacuum of space. This medium was known as the luminiferous aether, and 19th-century theorists believed it was necessary to support the propagation of light waves through the universe.
Early wave theories of light argued by analogy that light waves must have a medium through which to spread in the same way that sound waves do. The aether model was a theoretical effort to describe that medium. In a series of experiments, Albert Michelson (later joined by Edward Morley) attempted to measure the motion of the aether relative to the Earth. Their results were negative, demonstrating that light travels without an enabling medium and laying the groundwork for the later theory of Special Relativity.
The experiment involved splitting a beam of light inside a device called an interferometer. One beam of light was given a direct path to a receiving telescope, while the other beam was deflected by a series of mirrors. If the aether existed, the second beam would have been slowed down and would have arrived at the telescope after a noticeable delay. In fact, both beams arrived simultaneously, demonstrating that no medium existed to alter the speed of light.