Michelson and Morley used a large, sensitive spectrometer to compare the behaviour of light as it travelled along two paths at right angles to each other. As we saw un the introductory film clip, these results were vitally important for Einstein's theory of relativity.
The technical background, which is not necessary for the rest of the discussion, is this: The spectrometer uses the wave property of interference to compare the time that light takes to travel along the two paths. If it takes equal times, then the rays combine in step (technically: in phase) and the resultant beam is bright. If they are out of step by half a wavelength, then they cancel out, and the combined beam is dark (at the particular angle). Looking into the combined beam, what one sees is a series of bright and dark rings, corresponding to reinforcement or cancellation of the light rays that have travelled along the two different paths.
