The Marauder's Map is a magical map of Hogwarts created by Remus Lupin, Peter Pettigrew, Sirius Black, and James Potter (respectively nicknamed "Moony", "Wormtail", "Padfoot", and "Prongs") while they were students at Hogwarts. During this time, they gained extensive knowledge about the school grounds, such as its various hidden passages, from their frequent night-time adventures together.
In Harry Potter and the Prisoner of Azkaban, Fred and George Weasley give the map to Harry so he can travel to the village of Hogsmeade through a hidden passageway. The twins had previously stolen the map from a drawer in Filch's office that contained dangerous confiscated objects; it is revealed by Lupin that Filch probably knew what it was but not how to work it. Snape later finds the map in Harry's possession and tries to force it to reveal its secrets, but the map merely mocks and insults him. Lupin (one of the creators of the map), the current Defence Against the Dark Arts teacher, is called upon to investigate this "dark object", and confiscates it to keep Harry safe, though he returns it to Harry after resigning his post at Hogwarts. From then on, the map becomes one of Harry's most useful tools in his ongoing adventures.
At first glance, the Map is simply a blank piece of parchment; but when the user points his wand at the Map and says, "I solemnly swear that I am up to no good", the message "Messrs. Moony, Wormtail, Padfoot and Prongs, purveyors of aids to magical mischief-makers, are proud to present the Marauder's Map," and a detailed layout of Hogwarts appears. Saying, "Mischief managed!" while pointing one's wand on the parchment returns the map to its original blank state. The map displays the location of everyone within the castle and its grounds, and includes the location of secret passageways and instructions on how to access them. Several locations like the Room of Requirement and the Chamber of Secrets do not appear on the map, either as the Marauders did not have any knowledge of them, or, in the case of the former, they are not a fixed location. Furthermore, Animagus disguises, Polyjuice Potion, and Invisibility Cloaks cannot fool the map, as shown in Goblet of Fire when Barty Crouch, Jr., using a Polyjuice Potion to disguise himself as Moody,[HP4] and in Prisoner of Azkaban when Peter Pettigrew, who is supposed to be dead [HP3] but as an Animagus has transformed into a rat, are both displayed on the map. Read more on Wikpedia.org HERE.