The motion of the stars. Stars are moving together in groups. They belong together, because they are born at the same time and place. For a very long time they will move at the same rate, in and out the arms of the Milky Way. We can compare the stars in the Galaxy like cars on a highway.
Some cars have a slightly different motion.
They come and go on the highway. An outsider will always see cars, passing on the motorway. But they are all other cars. The stars we see today in the sky, are just travelling past us. Within thousands of years other stars will be travelling in the vicinity of our Sun.