5 Nights at Freddies is exactly what it says: A remake of the well-known game powered by GZDoom. For those unaware of the mod, a brief little summary: You (and/or your friends, up to 8 players supported) must stay alive in the mysterious 'Freddy Fazbear's Pizzeria'.
Hold out until the clock hits 6:00 to survive each night. Each night will progressively get harder and harder, you'll have to manage your power, the location and behavior of the 4 animatronics (and possibly more), and other events. The recreation is well done, top-notch quality and therefore the 700MB are for a reason. Check the download and further information from the official development thread.