https://www.gamespot.com/articles/fortnites-cube-keeps-rolling-or-whatever-it-is-cub/1100-6461873/

Fortnite's Season 5 is starting to draw to a close, which means another big world event is just around the corner to usher in a new reality for players. Whatever is coming is likely tied to the mysterious cube that's been rolling its way across the map, and it's still going. [Update: It's now made its way to Loot Lake, where it melted or sank, turning the water purple and giving it bounce properties like the cube itself.]

The glowing cube first appeared after a massive lightning storm in the desert. The rift closed but the cube remained. Standing near it recharges shields, and shooting at it bounces back and hits you instead. It has been moving every one hour and 43 minutes. Players have theorized that the symbols along the sides are an encoded message, but no one has deciphered it so far.