Among other things, he pictured lightning striking at both ends of a moving train. A person on the embankment might see the strikes as simultaneous, but to someone on the speeding train they would appear to have happened at different moments. Because the train is speeding forward, the light from the strike at the front of the train would reach him a moment before the light from the strike at the back of the train. From that he realized that simultaneity is relative to your state of motion, and from that he came up with the idea that there is no such thing as absolute time. Time is relative. Hence the special theory of relativity.