The Michelson-Morley experiment used an interferometer to detect a hypothetical light-propagating "ether" by measuring expected changes in light speed due to Earth's movement through it. It was anticipated that light's speed would vary based on its direction relative to Earth's velocity.
However, the experiment yielded a null result, indicating no measurable variation in light's speed, irrespective of equipment orientation relative to Earth's orbit. This contradicted the ether hypothesis and the belief that light speed depends on the source's motion. Consequently, it was concluded that light's speed is constant for all observers, irrespective of their motion or the source's. This pivotal discovery became a cornerstone of Einstein's special relativity, which posits the speed of light in a vacuum (c) as a universal constant. Thus, the Michelson-Morley experiment was instrumental in establishing the Special Theory of Relativity.