Why does time p slower to someone who is orbiting the earth near the speed of light?

I heard some kind of story like "there are two twins that are 20 years old, and one goes into a space ship with a stopwatch and the other stays on earth. The one in the spaceship orbits the earth near the speed of light, and once the stopwatch in that space ship reaches 5 years the twin in the spaceship lands back on earth and discovers that his twin died 50 years ago" So why does time travel slower for someone who is traveling at extreme speeds?