https://www.gamespot.com/articles/fortnite-shadow-stones-where-to-find-them-and-how-/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: Shadow Stones were absent for much of the first week, as Epic was forced to pull them due to an exploit. They've since returned, and just in time, as using them is one of the Week 2 challenges.]

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.