Slither.io is based off of the Web based game Slither.io hence the name, and is played by through a snake.
Information/How To Join
Right click the bow to join, and you will see two blocks forming your eyes and floating blocks flying out behind you forming your tail. This represents your "Snake" and to eat people with. On the right side of the screen you can see the leaderboard. This represents how big you are. As you gain mass, the numbers will go higher, and the blocks following you will increase in length as well. The goal of the game, just like the browser game Slither.io, is to gain as much mass as possible.
The objective of the game is to be the biggest player/snake in the lobby. On the side shows a scoreboard to show who has the most mass.
Once you join the game, you will see little tiny wool block entities on the ground. When you walk into one, you will gain some mass and some XP. You will also lose mass over time, mainly through boosting. You can "kill" anyone simply by making them run into your body. Just like in the online browser game Slither.io you can kill other players by having them run into your tail. The game is preferably played in F5 mode which allows you to watch your snake from above. This also allows you to see in a wider range around you, giving a tactival advantage.
There is technically no way to "win" other than be the person with the highest mass in the game as long as possible.