A space probe can use the momentum from the orbital motion of a planet in order to change its
velocity, in magnitude or direction. Using very precise trajectories a probe can approach a planet
from the opposite direction of its motion and be caught in orbit, accelerating due to the
gravitational field and the momentum of the planet moving in its orbit. When the probe leaves
that gravitational field it retains the momentum of the orbital motion (although it loses the velocity gained by the gravitational acceleration), and this allows it to travel
in a different direction and at a higher velocity without using as much fuel as otherwise needed.
This is useful for a probe attempting to travel to the outer planets, that is, away from the Sun.