One of the active quests in Fortnite Battle Royale is the Delta-One Set. There are five challenges in this quest, and one of them is to crouch behind a barrier.
Here's where you need to crouch behind a barrier in Fortnite.
The barriers that you need to crouch behind are labeled by their Gears of War logos. There are five scattered around the map, and you can find one at each of these locations:
Once you make it to one of these barriers, just walk up beside it and crouch by hitting the left bumper, or whatever corresponds to LB/L1 on your PC, and you're good to move on to the next quest. You could leave the lobby if you want to immediately begin your next one, or stay to finish the game, however you can only complete one quest per match.
The Delta One quests expire on Dec. 17, so get to it if you want to unlock the exclusive Crimson Omen Spray. You can also purchase Gears of War characters Marcus Fenix and Kait Diaz off of the Fortnite Store for 1,500 V-Bucks each, or as a Bundle for 2,000 V-Bucks.