Context helps.
The quote comes from "Mere Christianity," where Lewis is discussing his reasons for having been an atheist -- note that some readers have disputed whether he ever was an atheist, but let's take him at his word.
He says that he was an atheist because he felt that the universe was "unjust." But then he wondered: How do I know the universe is "unjust"? Because you can't call something "unjust" unless you have some idea of what justice is, any more than you can call a line "crooked" unless you've seen a straight line.
But here's the problem: in a world of only crooked lines, where would you find the idea of a straight one? Likewise, if the whole world is "unjust," where can you get an idea of "justice"?
You can see where this is going. We can't call the universe "meaningless" unless we have some idea of what "meaning" is; and if the universe is truly meaningless, where would that idea come from? The fact that we can conceive of the universe as "meaningless" therefore proves that it must have meaning.
Personally, I think he's playing word games. But that's his argument.