"Moony, Wormtail Padfoot, and Prongs ...We owe them so much."
-- George Weasley (PA10)
Fred and George Weasley know that Harry desperately wants to go to Hogsmeade but he is not allowed because escaped convict Sirius Black is on the loose and the castle is surrounded by Dementors. So they decide to give him the Marauder’s Map, which they discovered in Filch’s filing cabinet under “Confiscated and Most Dangerous“. It has the magical ability to show the names and locations of everyone in the castle, as well as seven secret passages. The Map was created by Harry’s father, James Potter and his friends who called themselves “Moony, Wormtail, Padfoot and Prongs“.
From the Web
Writing by J K Rowling on WizardingWorld (Pottermore): The Marauders' Map
Harry Potter Wiki: Marauder's Map