The Barrier is a large red wall that surrounds the area of gameplay. Upon contact, a snake will die with no remains left. It is also called the Edge.
Some players use the Barrier as a means to lure and destroy other Snakes. However you don't find a lot snakes around the barrier usually, unless they are in the North.
You can put over half of your body into the edge, making it easier to squeeze through narrow openings, as shown.
When you are sure you have no signs in surviving and you are near the Barrier, you can suicide into the Barrier by hitting the barrier where no mass would get left behind if you do so.
The Barrier is a circle with a red outline and a dark red background.