The Freddy Mask is a core game mechanic in Five Nights at Freddy's 2 that is the main means of keeping the Animatronics away (the others being the Flashlight against Foxy and the Music Box against The Puppet). It is an extra empty Freddy Fazbear head given to the new night shift security guard, Jeremy Fitzgerald, at Freddy Fazbear's Pizza. The player can use the head by hovering over the red tab at the bottom of the screen (next to the grey Monitor tab).
The mask was supplied after complaints were filed "about the characters trying to get into The Office" by the previous guard. It is used to fool any animatronic (excluding Foxy and The Puppet) that enters The Office into believing that the player is an animatronic themselves.