The monolith token allows a player to create a new settlement. Unlike other tokens, it can be picked up even when a player is not bound to a settlement and can be placed anywhere in the world. Once it is placed, a monolith statue is created, which becomes the center of the new settlement.
Monolith tokens are only available in the world when there are an insufficient number of settlements per active player.
At the time of writing that number is currently 1 settlement per 10 players. If there were 12 active players and only one settlement there would be one additional monolith token in the world.
When the player numbers allow another monolith the next token a player picks up will form into a monolith token when the player isn't currently carrying other tokens.