The Nickname Box is where the player types a name which they would like to use in the game.The name will then be placed in the center of their cell. By typing a specific name you can use a special skin instead of a normal color. It is not compulsory to fill this box out. If not filled, this will result in the name "An Unnamed Cell" if the player is on the leaderboard. Plus, nothing will be written in the player's cell. On mobile, it will appear as "Unnamed".
Some people, when spawn as a green cell or in the green team, leave the nickname box blank just to troll newbies, that can think the player is a virus, and try to hide into the cell, but are eaten instead. This strategy isn't useful againist virus farmers.