Pokémon Red and Green are set in the fictional region of Kanto.
Kanto is the first main-series Pokémon region and contains locations such as Pallet Town, Viridian City, Pewter City, and Saffron City. The player travels through the region while collecting Gym Badges and building a team.
The region's name was not used in the original Japanese games in the same way it later became established. Kanto became the standard name associated with the first game's map and was clearly identified in later Pokémon media and games.
Kanto is also based partly on the real-world Kantō region of Japan, which includes Tokyo and surrounding areas. That geographical connection is one reason Pokémon regions often draw on recognizable Japanese or other real-world landscapes. Kanto later returned in Pokémon Gold and Silver as an explorable post-game area.