Fear not, I will offer no spoilers if you haven't found time to read book 6 in the series. I would just say that you are missing out until you have read them all.
This line is sandwiched in a scene with a Marsh-wiggle and a Witch. (That line makes no sense to anyone who has never heard of Narnia.) The Witch is trying to convince the heroes, including the aforementioned marsh-wiggle, that the world they thought existed was only an illusion. The Marsh-wiggle, called Puddleglum, says the following:
One word, Ma'am. One word. All you've been saying is quite right, I shouldn't wonder. I'm a chap who always liked to know the worst and then put the best face I can on it. So I won't deny any of what you've said. But there's one thing more to be said, even so. Suppose we have only dreamed, or made up, all those things - trees and grass and sun and moon and stars and Aslan himself. Suppose we have. Then all I can say is that, in that case, the made-up things seem a good deal more important than the real ones. Suppose this black pit of a kingdom of yours is the only world. Well, it strikes me as a pretty poor one. And that's a funny thing, when you come to think of it. We're just babies making up a game, if you're right. But four babies playing a game can make a play-world which licks your real world hollow. That's why I'm going to stand by the play-world. I'm on Aslan's side even if there isn't any Aslan to lead it. I'm going to live as a Narnian as I can even if there isn't any Narnia. So, thanking you kindly for our supper, if these two gentlemen, and the young lady are ready, we're leaving your court at once and setting out in teh dark to spend our live slooking for Overland. Not that our lives will be very long, I should think; but that's small loss if te world's as dull a place as you say.
Well said, Puddleglum, well said indeed.
We're fed many lies in the world of today. So many, in fact, that more lies are built on top of other lies so that we don't even know where to begin. But the Truth is out there. I will chase after Him for as long as I have breath, until I can chase no longer. I will seek after it with everything i am until such a time as I find it.
I believe I am on the right path.