Take the set of all numbers between 0 and 1.
There are an infinite number of numbers in this set.
There is no biggest number in the set (any number that you specify will always have a higher number)
However, the set is bounded by 0 and 1.
1 is not a part of the set but stands outside of it.
He suggested that God is like the number 1. "He" exists outside the set of all our creational attempts to speak of "him" none of which can be identified with "him". But just as some numbers in the set of numbers between 0 and 1 are closer to 1 than others so too some God-talk bears a closer similarity to God than others. Nevertheless there is always an infinite gap between our talk of God and God-himself (because for any number between 0 and 1 there are an infinite number of numbers between that number and 1).
I thought that was quite an interesting analogy. It has its weaknesses (as all analogies do) but it possibly has something to contribute to the perennial discussion on human conceptions and the Creator God.