A very simple way to explain it: imagine you are on a playground and there is a merry go round right in front of you. A kid jumps in it and it starts spinning.

A very simple way to explain it: imagine you are on a playground and there is a merry go round right in front of you. A kid jumps in it and it starts spinning. At the same time you start to slowly walk away in a straight line. If you keep looking at him, you will see that the distance between you two varies (he gets closer, then farther, then closer back again) due to the rotation of the merry go round. In this example, the merry go round is the solar system, the Sun is at its center, fixed in this frame of reference, the kid is the Earth and you are the Voyager probe. Hope it helps!