It may be hard to believe, but some people on earth still don’t know about “Fortnite: Battle Royale.”

It’s a shoot-‘em-up game, and holy moly is it huge. “Fortnite” grew to 125 million players less than a year after its launch, its developer, Epic Games, said last month. In February, “Fortnite” hit a peak of 3.4 million simultaneous players, and the company made a record $318 million in May.

Celebrities like Drake are playing it; people like Tyler Blevins (better known as Ninja) are becoming celebrities by playing it.

“Fortnite” is affecting how kids talk, how other games are designed, and how sports players celebrate.

The “Fortnite” craze is disrupting school, addicting pro basketball and football players, and worrying parents.

Want to understand what all the fuss is about? Read on.

The game

“Fortnite” is a PVP, or player-versus player game. (The full name is “Fortnite: Battle Royale” It’s a spinoff of an earlier game, now called “Fortnite: Save the World.” And while we’re talking about genealogy: “Fortnite” borrows much of its look and game play from last year’s hit PVP, called “Player Unknown’s: Battlegrounds,” or PUBG for short.)

In “Fortnite,” you drop onto an island with 99 other players. You scrounge for weapons, you build things for protection, like shields and forts (echoes of “Minecraft”), and you shoot other players. The last person alive is the winner.

It takes only 20 minutes to play a round, assuming you live that long, which makes it snackable and convenient.

During that time, a steadily shrinking circle of shimmering blue contracts over the map of your island. It pushes anyone still alive closer and closer together as the game goes on, adding to the excitement.

The popularity

The screaming, white-hot success of “Fortnite” stems mostly from the fact that it’s free to play and available on every platform (Mac, Windows, iPhone, Xbox, Playstation, Switch, and, soon, Android). In other words, there are hardly any barriers to getting all of your friends in on the game. (Xbox and Playstation people can’t play each other, but everyone else can play together.)

You can make in-app purchases, but none of it affects the game play. It’s stuff like skins—different looks for your character—and funny victory dances. (Those are the dances we’re now seeing on professional sports playing fields when someone scores a goal.)

You can also buy a Battle Pass, which lets you earn more in-game goodies by completing challenges along the way. It costs $10 per season, which lasts about 10 weeks.

More reasons for the success of “Fortnite:”