“Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (reference-frame of the train). Every reference-body has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.”Albert Einstein · Relativity: The Special and General Theory · 1916 · Relativity: The Special and General Theory, cap. IX
Time is not universal: every reference frame has its own time, and mass and speed stretch it.
Picture space and time as a stretched elastic sheet. A heavy planet bends it, just like a heavy ball bends a stretched blanket. Other objects passing by slide along that curve, which we call gravity. The heavier the mass, the stronger the curve — and the stronger the effect on time.
If you put one clock at your feet and another exactly at the top of your head, the lower one would tick slower. The difference is tiny, but real. Earth pulls harder on the lower clock, and strong gravity slows time down. So your feet age more slowly than your head. It is a minuscule effect, but measurable. Now think about what happens when gravity is even stronger.
That is why GPS satellites must correct their clocks. They move fast, and speed slows down time. But they are also farther from Earth, where gravity is weaker, so time speeds up. The two effects oppose each other. Without adjustments, your phone would see your position a few kilometers behind. It is like reading a map drawn yesterday, but the streets moved today. Now imagine a clock near a black hole.
Near a black hole, time almost stops. If you stood at its edge and watched someone fall in, you would see them frozen in motion. For the one falling, however, everything seems normal. Only that, for them, your seconds become hours. When they returned, you would have aged years, they minutes. It is like waiting for a friend in a lift that never comes — but this time the lift is the universe itself. Now imagine what a journey to a distant star would look like.
Why it matters Without this correction, the GPS in your phone would be wrong by kilometers every day.