The Mirror of Erised is the magical mirror where it becomes the plot point in the film Harry Potter and the Philosopher's Stone.
It is first seen in a room where Harry discovered his biological parents in the reflection. He shows Ron of this, and only Ron sees is him holding the Quidditch Cup. The next night, Harry visited the mirror, until Dumbledore arrives. He explains all about the mirror and reveals to Harry how he never knew of his family, before telling him how men became obsessed with it, prompting Dumbledore to plan to move it to a another room, advising Harry against looking for it.
The mirror is moved to the Underground Chambers and Dumbledore places The Philosopher's Stone inside to protect it. Snape is to have thought of being involved in the plot to steal the stone, Harry, Ron and Hermione plan to stop him. After passing through a few obstacles, Ron is knocked out and Hermione stays with him as Harry goes on alone.
It is revealed that Quirrell is involved in the plot all along. Quirrell had failed to obtain the stone from the mirror until he forces Harry to see in the mirror; Harry discovers that the stone has magically appeared in his pocket. When Voldemort is revealed by Quirrell after he unravels his turban, he tries to manipulate Harry into giving him the stone in order to bring his parents back from the dead. When Harry refuses, the final showdown ensues, ending with Quirrell being disintegrated into dust and ashes after being burned by Harry's touch (due to his mother's love-based protection), and Harry is knocked out by Voldemort's spirit. Dumbledore rescued Harry from the chambers and had the stone destroyed. It is unknown what happened to the mirror. It might have stayed in the chamber or returned to its classroom.