The Marauder's Map is a magical map made by James Potter, Sirius Black, Remus Lupin, and Peter Pettigrew in their school days. It shows every single room in Hogwarts, along with corridors. It even shows hidden passageways that they discovered themselves. Most remarkable of all, when looking at the map, you see little dots that depicts everyone in the castle and where they are.
Fred Weasley, along with his twin, George Weasley, steal it from Argus Filch the Hogwarts caretaker. Filch had conviscated it years ago and now Fred and George had it. In Harry Potter and the Prisoner of Azkaban, Fred and George give Harry the map, saying that the have no need for it anymore. Harry uses it quite regularly throughout his school years.
To activate the map, you must tap it with your wand and say "I solemnly swear that I am up to no good."
To make the map clear iself, just tap it with your wand and say "Mischief managed."