There's a trend of "io games" which are so named because the first one was hosted on a site called "agar.io". then was followed by slither.io and so on. They are characterized by a few distinctive qualities: they're entirely browser-based, players are instantly dropped into a big lobby with tons of other players, they typically have simplistic graphics (e.g. powered by canvas), they have no long-term progression (once you die you restart from the beginning), and they have some kind of leaderboard where you can see the players in your shard that have progressed the farthest since the last time they died.

It's just a browser based multi-player game with a .io TLD. Hope to make a Show HN soon :)