The sled first comes up when The Giver compares the process of receiving the memories to sliding downhill in the snow. At first it's all birthday parties and wind in your face but, before you know it, your arm's being blown to pieces (which, in his comparison, is like snow piling up on your runners. Right…). Jonas then gets to experience this himself, as the sled is his first memory. Of course, we see the same sled again at the end of the novel, as Jonas races downhill toward the village that's either a hallucination or the Elsewhere of his dreams.
Because The Giver explicitly spells out the simile for you, there's not much more to say in the vein of "riding downhill = receiving memories." But we can think about what it means when Jonas actually does ride down the hill at the end of the novel. Until now, this action has been a sort of dream, someone else's memory, someone else's exhilaration, someone else's difficulty and pain. But now all of these things are very much Jonas's own. The novel even states explicitly that now Jonas is using his own recollections, rather than drawing on the fleeting memories passed on to him by The Giver. In short, his dreams have become reality, and the sled clues us into that. Of course, it's also possible that the final sled ride is just Jonas's memory, and that it's not happening at all, which would really shoot that theory down.