Slither.io is a multiplayer online game where you'll have to eat as many marbles as possible to make your worm grow.
It is basically an improved version of the snake game (from old cell phones) where you control a worm and you must eat the colored balls to get bigger, every time you die, you generate more balls that other online players will eat to grow their respective earthworms.
It's a very simple game to play, you just need to control your worm and pay attention not to bump into other players, and of course, eat a lot to make your worm grow.