A game masters screen (Dungeon master's screen, storyteller's screen, whatever it's called) is a table top role playing game accessory which the game master puts in front of his place at the table. Typical game master screens include a player facing side depicting art reminiscent of the game. The game master facing side has quick reference material for the game generally in the form of several tables. There are multiple purposes for these screens: they give the GM a modicum more gravitas, they make necessary info instantly available, and they conceal the Game Master notes, maps, and dice rolls for outcomes that the players shouldn't know. None of those is strictly necessary for game mastering but most of it is desirable. In the standard dungeon crawl the DM is going to need a map as reference and it wouldn't do to have the players see it. Opening the core book to confirm some number can kill the pace of play.
There are two and a half kinds of GM screens. The first is a game line specific cardboard screen. It's basically a game board except that it's four panels that fold accordion wise. Art work faces the players, tables the game master. These tend to cost about twenty to thirty dollars. These are good if you are only ever going to play that one game or just love the company that you feel the need to have one of everything they sell. The other option is to buy a two to five dollar PDF copy of the pages of the screen and put them in a blank screen. Generic game master screens are typically four portrait oriented heavy cardboard panels with clear plastic sheaths on the front and back to allow for the insertion of printed pages. These are the two major expressions of the concept. Much more complicated and expensive options exist like the hand carved wood screens with magnets to hold pages on . TTRPGs being a do-it-yourself, hobbyist's space many options have been explored. If you don't mind going full ghetto just steal a restaurant menu and put pages you printed yourself in it. The player who gives you crap about it can be the one who's class abilities never get used that session.
After polyhedral dice the game master's screen is the most recognizable element of the hobby and acts like a badge of office for game runners. The ideal screen is an exercise in brevity that provides exactly what you need and nothing that you don't. For instance a good screen for 5th edition D&D would list all of the conditions and what they do or each level of exhaustion but wouldn't list the xp to levels or how hard a DC 10 check is. The first two are things that would be easy to misremember and are needed in play while the third shouldn't come up more than once every few sessions and the last is so basic that nobody running the game should need it explained. Some game masters won't want a screen as it creates visual and psychological distance between them and the players. Game Mastering is one of those tasks that is simultaneously taxing and rewarding because of the terrific level of responsibility and freedom. You are the world that is happening to the players and their characters and the screen physically instantiates that separation for better or worse.
IRON NODER XVI: MORE STUBBORN-HARD THAN HAMMER'D IRON