What are Pokémon and why is everyone playing this game?
The world was introduced to Pokémon in 1996 when the first games came out. There were Pokémon Red and Pokémon Blue for the original Game Boy. The idea was that there are Pokémon all over the world and children of 10 get the chance to go out and explore. They have Pokéballs to capture the (some not so little) monsters and fit them in their pocket, hence PocketMonster or Pokémon. So a generation was introduced to these Pokémon. The popularity was so immense that spinoffs started to happen, like a manga tv-series, books, more games, more series, and so on.
151 Pokémon could be captured in the first wave (more were introduced).
So why is it so popular? Because it speaks to more a whole generation. They have either lost themselves in studies or jobs or relations, but rarely do you hear them talking about the new Pokémon Sun and Moon games, or with a little bit of negativity because "they are so different from our first generation of Pokémon". But Niantic Labs (the Pokémon company) re-introduced said first generation. And in a very special way, that is. Now we ourselves could go out and explore the world. Find all those rare Pokémon. And of course be the very best.