Here is how the feature works. When you enter the store of objects in Fortnite and select an item to buy, two options are now available: buy it for yourself or "Buy as a gift". Select this and Fortnite will provide a second screen on which you can select a friend to send the article to. You can add a custom message if you wish. Then send it and your friend will receive a gift box containing the article inside the next time he will launch the party.
It should be noted that you can not offer any gifts to anyone. Epic Games has put in place a system of requirements designed to prevent people from hijacking an account, buying many items and sending them to their other account. First, you need to enable multi-factor authentication on your account to send gifts. You can also send gifts to people with whom you are friends for at least 48 hours. Finally, you can only give three gifts over a 24-hour period.
Once you have decided to offer a gift, the purchase is not refundable and you can only offer items available in the shop. If you are more a donor than a recipient, you can disable the possibility of getting gifts in your account settings.