A d20 RPG is a “Pen and Paper” tabletop game typically played with a group of 2-8 players. The name comes from the twenty sided die(the “d20”) used in the game to check for success or failure. Players gather together to collectively tell a story powered by their imagination and driven by a the dice.
You’ve almost certainly heard of at least one d20 RPG game. The most well known is the original game Dungeons and Dragons. D&D, as it is known, was developed in the ’70s by Gary Gygax and David Arneson. It is currently up to it’s 5th official edition to critical and fan acclaim. When wizards published the 3rd edition and released the rules which the game runs on under the OGL they started the “Pen and Paper Renaissance”.