This challenge is part of the Brutus' Briefing, the first agent on which Fortnite's Chapter 2 Season 2 focuses. You'll have to carry a knocked opponent over 50m. The feature allowing you to carry knockout people was incorporated into Epic Games' battle royale at the launch of Chapter 2 Season 1. It's most often used to move an ally in a bad position, but it can also be used to move an enemy quickly, or to throw him from a great height.
You'll have to run 50m with an opponent on your back. That's a pretty big distance, so make sure the area is clear before you start your sprint!
Duos mode is recommended for this challenge.