The phenomenon – first theorized by Albert Einstein – affects the rate at which time passes for objects moving at different speeds. What does this have to do with GPS? The answer relates to how orbits work.
Imagine what it must have been like to try to navigate before satellites and computers before even compasses were invented. For thousands of years, the primary means of navigation was to track the stars and compare their position in the sky to determine where you were. Accuracy was limited.