It's not exactly in my mind, but when the kids returned from Narnia, they said something like "you won't believe it" to their uncle. But, the uncle (the owner of the mansion) did not seem surprised, leading me to believe that he had experienced it before (probably, when he was kid).
In Narnia, there was a prophecy that the kids would come to rule the world. What parts were the uncle fit in?
This man is Digory Kirke, and while I don't believe it's been specified in the movie universe, in the original novels Digory is one of two protagonists in the first Narnia novel (chronologically), The Magician's Nephew.
As a child he and Polly end up in Narnia, and are two of the first, if not the first humans to do so. They witness Aslan creating the land.
They, if my memory serves, are also responsible for bringing the White Witch, called Jadis here, into Narnia, as well as Frank the cab driver (who is made the First King of Narnia) and his cab horse, who becomes a talking animal.
Though published sixth in the series, this novel is chronologically first (as a prequel) and explains things such as the lamppost in Narnia; and in some collections is presented first.
As DavidS points out (thanks muchly), the children take an apple with them from Narnia back to Earth, which is planted (along with, as MartianInvader pointed out [ta!], the magic world-hopping rings that give the wood a magical nature) and grows into a tree, from which this wardrobe is created (This is very much a "this is why" installment to the series).
In addition (and we're sliding a tad away from the point here), earlier in the movie the other children complain about Lucy's stories to Uncle Digory, who in so many words tells them they should be more believing, and (I believe) outright saying that he believes her.
Digory's story is very much internal. His mother is sick, dying. He brings the witch to Narnia by mistake and Aslan gives him a quest to atone for it. The witch tries to persuade him to steal a magic apple which can grant him immortality, or heal his mother (she also tries to make him abandon Polly). He doesn't, thankfully since the magic is your standard tricksy be careful what you wish for stuff. As thanks Aslan gives him a new apple which does heal his mother, and then he uses the seeds to plant the tree.
However, because of his actions humans came to Narnia and a portal was made between our world and Narnia. He didn't do much of importance during his brief stay, but because of his actions a human was there to be made king, and his actions also provide Lucy, Edmund, Susan and Peter a way to get into Narnia
