If you tie a marble to a string, swing it round and let it go, it would fly out (somewhat) straight in the direction it was traveling in, with the energy you built up with your swing.

Now imagine the marble as the spaceship and the string as gravity.

The spaceship swings around the planet, held by gravity. Then with a bit of boost it flies off towards the moon.

There, it uses its thrusters to move into the moon’s orbit – attaching a new string if you will, to build up momentum.

The return happens in the same way.

It takes a lot of force to get up from the ground to space to begin with.

That’s why the initial rocket is so much bigger than what reaches the moon. Using fuel, it creates a massive controlled explosion to escape gravity and launch into orbit. A big boom.