>>20474817It's Pokémon as an Internet game; you can go through any of the areas in the main games and collect Pokés, grow berries, do text adventure dungeons, trade with other players, etc.
Basically, at its core, it's a collectibles game. The Mew is gold because it's from a special event. Many of the site's events introduce palette-swapped Pokés or original/fun sprites. It's a fun thing to play casually. Really good for passing time when you can't invest tons of it at a time.