Climb to the top. Reactive: Indicates your current altitude.
Altitude backpack is a Legendary Fortnite Backpack from the Zenith set. Back bling Altitude is reactive. It means it changes or evolves (reacts) depending on certain things in game (damage dealt to opponents, if it's day or night...)
The Set Zenith contains 2 cosmetic items.
The backbling Altitude belongs to Chapter 1 Season 7. It's not yet known how to get the backbling Altitude.