The effect known as the gravity assist or slingshot effect is a way of using the motion of a planet to accelerate a space probe on its journey towards the outer planets.
As the space probe moves towards the planet it will accelerate but because Jupiter is moving it is “dragged along” by the planet, having its speed increased considerably by the planets motion. Its speed after the encounter is much greater than it was before. The gravity pull of the planet has given the space probe some additional energy.
Of course the gravity pull of the space probe will have slowed down Jupiter but by only a tiny amount because the planet is so much more massive than the space probe.