https://www.gamespot.com/articles/how-fortnite-season-6s-shadow-stones-work-and-what/1100-6462108/

One of the biggest changes added to Fortnite in Season 6 is the introduction of a new consumable called Shadow Stones. Using one allows you to transform into your Shadow Form, which gifts you with a couple of supernatural powers. [Update: Not long after they were added, Epic pulled Shadow Stones from the game entirely due to an unspecified bug. They were then re-introduced but have since been pulled again. There's no word on when they will return.]

In Shadow Form, you won't be able to use weapons, but whenever you stand still, you'll go completely invisible. You'll still be slightly visible whenever you move in Shadow Form, and walking around leaves behind a Shadow Trail that people can easily follow. Running around isn't all that bad though, as you'll have increased movement speed and jump height, and you won't take fall damage.