I'm now in a general cosepts about orbits. I have a question, what the book says in here?

I can't understand theFrom Equation 2.24, it is clear that as the distance, r, between the rocket and Earth gets larger and larger, the potential energy between them decreases. At a certain distance, the effect of the gravitational potential well of the Earth becomes neglible compared to other forces acting on it (such as gravitational pull of the Moon or the Sun) and the rocket is said to be out of the Earth's "sphere of influence". The term is only applicable for three or more body systems and can be dscribed as,

r_{SOI}= α_{p}(m_{smallerbody}/M_{largerbody})^{2/5}

Here, α_{p}is the length of the semimajor axis of the smaller body's orbit relative to the larger body.

**bold**formatted sentence. What the author try to mean? If you can, can you describe this sentence by using images, datas or diagrams, please?

Thanks to everybody, have a good day!