Aslan's Country is a mystical place full of mystery, peace and prosperity; similar to that of Heaven, and the afterlife. It is where the almighty lion Aslan himself came from, where those who have died go to "live" together in eternal tranquility and freedom.
It is located at the very end of the world itself, and was first mentioned in The Chronicles of Narnia:The Voyage of the Dawn Treader by Reepicheep and a curious Lucy. The brave mouse said that he had always longed to see the end of the world and where the great lion came from before actually arriving in Narnia.
At the end of the movie, Reepicheep knees before Aslan and expresses his lifelong desire to see Aslan's country with his own eyes. Aslan agreed, but warned that once entered, one could not return. The mouse accepted and said his farewells to Lucy, Edmund, Caspian, and his new companion Eustace; to whom he admitted to have enjoyed fighting alongside such a brave warrior, and great friend.
Its main entrance lies far beyond the vast sea of Narnia.