What does it mean to say that the speed of light is a constant?

Saying that the speed of light is a constant means that light travels at the same speed, approximately 299,792 kilometers per second (or about 186,282 miles per second), in a vacuum, regardless of the observer's motion or the source of the light. This constancy is a fundamental principle of Einstein's theory of relativity, influencing our understanding of space, time, and the structure of the universe. It implies that the laws of physics are the same for all observers, leading to phenomena like time dilation and length contraction.