Slither.io is an evolved version of the old videogame classic Blockade from 1976, which introduced the concept of Snake.
In Slither you start as a small snake, your objective is to get as big as possible by eating the colored globes, and at best eliminating other players.
How to play Slither.io
Your worm moves automatically at any time, you control its direction by moving your mouse cursor around the screen.
Hold down your Mouse 1 button to activate sprint, it comes at the cost of your size, when you sprint you drop small colored globes behind you.
To eliminate other players, you have to pass in front of their path, by blocking them you effectively kill off their snake, which turns their whole snake into colored globes for to pick up.
The bigger your snake becomes the slower it also gets, beware of the fast small ones.