https://www.gamespot.com/articles/fortnites-playground-mode-update-is-live-but-you-c/1100-6460056/

Fortnite's latest update, v4.5, is now available on all platforms. On top of introducing a new weapon and other features to both Battle Royale and Save the World, it marks the start of the game's latest limited-time mode, Playground--although as of this writing, the mode has been temporarily taken offline.

Unlike previous limited-time modes, Playground is intended to be a "low-pressure environment" that acts much like a private server. It allows players to explore the map, devise strategies, and play around freely with up to three friends without the risk of being eliminated by other players.