Beacon is an entity that was supposed to be in Agar.io.
Even though the developer of Agar, Zeach, didn't release the Beacon to the public, he did develop it and he shared the image on Reddit.
Beacon is something similar to the spawner cell. When a player feeds it, it gives the player back rainbow pellets.
If it is bigger than the player, it won't swallow the player, but also if the player is bigger than it, the player can't swallow it.
Some private servers use Zeach's JS code of a Beacon. There is even a YouTube video showcasing it.