I like the explanation provided in this Discover article.

""For us believing physicists," Einstein once wrote, "the distinction between the past, the present, and the future is only an illusion." It was a melancholy remark, made as Einstein faced death, but it flowed directly from Einstein's special theory of relativity. Imagine a group of observers scattered carelessly throughout the cosmos. Each is able to organize the events of his life into a linear order—a world line of the kind just described. Each is convinced that his life consists of a series of nows, moving moments passing from the past to the present to the future. Special relativity urges a contrary claim. The observers scattered throughout space and time are all convinced their sense of now is universal. Now is, after all, now, is it not? Apparently not. Time passes at a different rate depending on how fast a person is moving: While one hour passes on Earth, only a few seconds might pass on a rocket ship hurtling away from Earth at nearly the speed of light. It is entirely possible that one man's now might be another man's past or future. "