Finding the minimum product of two numbers whose difference is 8, HOW?

This is the solution in my textbook:

"The quantity to be minimized is the product, p, of two numbers.

Let one number be n.

Since the difference between the two numbers is 8, the other number can be represented by n-8 or n+8.

Either expression can be used. We will use n-8.

p = n * (n-8)

...

"

Now the thing I don't get is that why do we need P? Why do we multiply the two numbers?