Tall Grass is a game mechanic found in nearly every Pokémon game. Pokémon supposedly leap out of the tall grass. Patches of grass can be found on some routes and towns in the Pokémon world where wild Pokémon can be captured. Tall grass is found in numerous places, including a few towns and cities.
Tall grass is always the first form of wild Pokémon encounter location seen in every game (assuming one doesn't count the water south of Pallet Town in Kanto). Often, the first "route" is designed to force the player to battle at least one wild Pokémon on the way to the second town. Tall grass is not, however, the most abundant location for finding Pokémon in every game.
In Generation I, II, and III, tall grass can be cleared by using the move Cut; doing so will clear a 2-by-2 square (in Generation III it's a 3x3 square or, with the Hyper Cutter Ability, a 5x5 square) of it, which will grow back once you leave the route.