The Captain Marvel Skin is a Marvel Fortnite Outfit. This cosmetic has been leaked, and the skin will hopefully be available to purchase in the near future!
Captain Marvel has not yet been confirmed as a skin, but there's strong evidence because she's shown in the Fortnite x Avengers comic book which is shown below. This also features Ghost Rider and Black Panther, and there's also the likelihood that Daredevil and Venom will be available.
How-to Get the Captain Marvel Skin
Captain Marvel was leaked or datamined, which means we don't know for sure how it will be released. Most cosmetics that are leaked will be released within 30 - 60 days of their discovery.