Since you have tagged your question under the game category, I'm going to assume that you are referring to the game (and website) slither.io.
Slither.io is a popular online multiplayer game in which players take on the role of a worm trying to grow and become the biggest worm (as measured by length, which is also your score). Slither.io games/servers will typically have several hundred players on at the same time, so when you enter a game you will have plenty of competition to deal with. A leaderboard of the top 10 players currently in your game is displayed in the upper right corner of your screen, and if you reach #1 amongst all players at the end of a day, then you can have a message displayed to all of Slither.io's players.
While playing the game, you grow your worm by collecting pellets, which can either be spawned naturally by the game or generated when a worm dies. Pellets coming from a dead worm tend to shine more brightly than normal pellets, and also award more length.
You can also give yourself a temporary speed boost (how to do it will vary depending on if you're playing in a browser or on a mobile device) at a cost of some of your length. The length lost in conducting a speed boost will appear as a trail of pellets behind your worm, so it is possible to recover that lost length later, if another player hasn't eaten the pellets first.
The most important rules are: (1) once you begin a game, you can never stop moving, and (2) if your head runs into any barrier (another worm or the edge of the map), you die and have to start over. Because of (2), the strategy for growing big quickly is to kill other worms (or be near a location where a lot of worm-killing is going on) and pick up the pellets that are left behind.
I hope that my answer gave you the information that you were looking for.