Kanto was the region name introduced for the original Pokémon games only in Generation II. The first-generation games take place in that region, but they do not use the name Kanto within their original in-game presentation.
Pokémon Red, Green, Blue, and Yellow feature the same broad setting: towns, routes, caves, forests, and the Pokémon League associated with the first 151 species. However, the region’s name was supplied later, when Pokémon Gold and Silver expanded the world and allowed players to return to the first games’ area.
Generation II introduced Johto as its main region and then presented Kanto as a second explorable area after the main story. This made the name meaningful in the games and clarified the geography of the earlier adventure.
Kanto is sometimes mistaken for a name used from the beginning because it is now inseparable from the first generation. The distinction is historical: the setting existed first, while the explicit regional name came later.