You need a lot of speed to go into space really fast.
When you fly your spacecraft really close to a planet and let it get influenced by its gravity, you let the planet transfer some of its massive amounts of kinetic energy into your spacecraft, which can add to its kinetic energy (and also help the spacecraft change direction)
You're essentially stealing some kinetic energy from planets to sling your spacecraft faster.
When you fly your spacecraft really close to a planet and let it get influenced by its gravity, you let the planet transfer some of its massive amounts of kinetic energy into your spacecraft, which can add to its kinetic energy (and also help the spacecraft change direction)
You're essentially stealing some kinetic energy from planets to sling your spacecraft faster.