Even though the rainbow is a nice move, this is definitely not a move you should focus on when you start playing soccer.
The rainbow is a very advanced move and it's really used only in rare cases of the game. It happens maybe once in a lifetime.
But the move, itself, means that the rainbow, the rainbow means to get the ball over the back of your head, coming forward, bouncing in front of you, and to keep moving forward.
So ideally you do the whole move over your own head but also over the head of your defender.
Meaning that you make one step in front of the ball, trapping the ball with the back foot, with the inside of the back foot and the heel of your front foot.
Once you move forward, you shift the body weight towards the front leg and roll the ball up with the back foot on to the heel of the front leg.
So it's complicated, but this is the way it's supposed to be.
So, the front leg. Again, the front leg will step in front of the ball while the back leg is trapping the ball and rolling it up the front leg.
At the same time you try to lean forward and move forward and kick the back leg up to hit the ball with the heel, over your head so that it comes down in front of you.
And that's the basics on doing the rainbow.