Pokémon Red Version was the very first Pokémon game and it was released for the original black and white Game Boy in 1996.
Like all subsequent Pokémon games it came as a pair of releases, so that players could have Pokémon exclusive to their version of the game and so that trading was required between versions – if you wanted to catch every single available Pokémon. Some might view that as cynical, but it wasn’t really intended to make people buy both versions, just to encourage link-up play and trading between them. It does however mean that you can’t catch all the available Pokémon if you only have one version of the game, and have no way of trading with someone else who has the other version.
In Japan, Pokémon Red (originally titled Pocket Monsters: Red) was accompanied by Pokémon Green (Pocket Monsters: Green), but in North America and Europe Pokémon Red was accompanied by Pokémon Blue, which is basically a remake of Green.