As you know, planets circle the sun and the sun holds them in a gravitational grip. If the planets didn’t have their own momentum, they would fall directly into the sun. But since they do have momentum (quality of any object with mass and velocity), they do not. They are in a perpetual fall towards the sun, but never really fall to it. If they would slow down, planets would fall into the sun, and if they went faster, they would shoot off themselves into deep space.
Note: the Sun is much more massive than the Earth, so the planets don’t fall down (to Earth) because they are orbiting the Sun, not the Earth.