A 4-sided die is a pyramid. A 2-sided die is a coin. What's a 1-sided die? It's a Möbius strip. No matter how you throw it, the die will always land on its only side. Awesome Dice developed it for role-playing games, such as Dungeons & Dragons.
Now to the obvious question: why would you ever need a 1-sided die?
It's a way of resolving conflicts that come up in gaming sessions. Sometimes there are argumentative players who press a game master to randomly roll outcomes rather than verbally lead the players along a storyline. When you have such a player in your group, get out your 1-sided die.