How do spacecraft use the Sun and planets to save fuel?

A gravity assist, or slingshot, happens when a spacecraft passes close to a planet and exchanges momentum with it. The planet loses an immeasurably tiny amount of momentum, while the spacecraft can gain speed, lose speed, or change direction depending on the trajectory. This technique is widely used because it saves a large amount of propellant and can make ambitious missions possible. The Voyager missions famously used multiple planetary flybys, and later missions have also relied on gravity assists to reach distant targets. Careful planning is essential because the timing and geometry must be extremely precise.