A preview is an advance of something that has not yet been released to the general public in its full form.
Explanation:
A preview of harry potter and the sorcerers stone would be the following:
Harry Potter lives with his uncles who treat him like a servant and make him live in poor conditions. On his birthday, Harry learns that his parents are wizards, and that therefore he has inherited magical powers. He must attend a famous school of magic and sorcery, where he makes friends with two young men who will become his companions on his adventure. During his first year at Hogwarts, he discovers that a malevolent and powerful wizard named Voldemort is in search of a sorcerer's stone that lengthens the life of whoever owns it. His mission is to stop this wizard and along the way he will discover shocking things about him, his past and his relationship with Voldemort.
