It's an effect given by an item called Nitro Splash (Rare/Blue) in Fortnite Chapter 5 Season 3 lasting from May 24th 2024 to August 15th of that same year. It has been nerfed multiple times with changes nobody has noticed. Once used, it gives you effects such as Infinite Sprint (used to be not Infinite but longer), Ability to walk on water, faster speed in general, faster reload speed and being able to be used on your teammates when they're next to you. You can also run into people with it and structures, dealing 90 (prev. 300) structure damage and 25 (prev. 75) damage to players. If you were in a boss car, it permanently had nitro as well as you would have permanent nitro if you had Megalo Don's medallion. If you used a boss car,you should notice that the firerate of your weapons and the turrets is increased by about 25%. It was vaulted at the beginning of season 4 as the Nitro production centre (AKA Redline Rig) was blown up by Doom in the Season 3 mini event.
You splashy splashy this funni juice on your frends while they make explicit remarks!!!1!!11!11 Fortnite Nitro does cool stuff !