RE: Remix The Finale

"noun
1.a thing that is helpful or beneficial.
"the navigation system will be a boon to both civilian and military users"
Similar: blessing, godsend, bonus, good thing, benefit, help, aid, advantage, gain, asset, privilege, luxury, windfall, bonanza, stroke of luck, , piece of good fortune, perk, plus, plus point, pro, perquisite, benison. Opposite: curse, disadvantage,
2.archaic
a favor or request.
"may I have the inestimable boon of a few minutes' conversation?""

It's basically a power up like Healing in the water or revealing enemy locations. They seem to be cards coming out of the Elemental chests that have appeared this season.

https://fortnite.fandom.com/wiki/Boons