Monster of the Week is a tabletop role-playing game centered around players solving mysteries and fighting monsters. The mechanics of the game are meant to mimic serialized TV shows like the X-Files and Buffy the Vampire Slayer. Each game session acts like one "episode" of a series.
Monster of the Week is a Powered by the Apocalypse (PbtA) game. This article explains what PbtA games are and where they came from.
The basic things you'll need for a MotW game are a copy of the rules, a place to print out game materials, and a couple 6-sided dice. If you decide to play online, you don't even need the printer.