Games are better when played with others. That is not a surprise to most, but is also the conclusion you can draw from comparing Slither.io to the original Snake.
In the game, you use your mouse to guide your snake around a playing field which is full of other players. You collect mass, in the same way as in the original game, to grow. When you kill another player, all their mass is left behind for you to pick up. But be quick, because another player might try to beat you!