They are real numbers, for which we have just proven it is impossible to find a description that will match them. We have proven that no description will ever describe them.
Of course you can describe them. But describe them in terms of what? I think that's the key. Let's say you have a pencil and you want to describe its length, which will be a unit multiplied by a number. If I were to try to describe the true length in meters it will be: 0.0178(...) * meters. This would be an "indescribable" number the author talks about.
However, I can just describe it in terms of itself and call it one pencil long - we use the pencil itself as the basis of the unit so the number we multiply the unit to is just: 1 * pencil long.
Of course you can describe them. But describe them in terms of what? I think that's the key. Let's say you have a pencil and you want to describe its length, which will be a unit multiplied by a number. If I were to try to describe the true length in meters it will be: 0.0178(...) * meters. This would be an "indescribable" number the author talks about.
However, I can just describe it in terms of itself and call it one pencil long - we use the pencil itself as the basis of the unit so the number we multiply the unit to is just: 1 * pencil long.