To get Supercharged XP in Fortnite Chapter 3 Season 1, you need to avoid playing Fortnite for a period of time. This is typically a day, and it sometimes also activates if you do play, but don’t complete your Daily Quests. This being Fortnite, the rules and conditions are subject to change. If you don’t play Fortnite every day, then you’re going to get a lot of Supercharged XP without even trying anyway.
Supercharged XP dramatically increases the amount of XP you earn, up until you’ve earned as much bonus XP as you missed by not completing the previous Daily Quests. Generally, it’s quicker to rack up XP through the Supercharged XP bonus than it is to complete all Daily Quests, so in the long run, deliberately avoiding Daily Quests and getting Supercharged XP is an effective way to level up fast.
You’ll know when you have Supercharged XP because your XP bar, which pops up whenever you earn XP, will be yellow instead of purple, and will be labelled “Supercharged XP”. Once it’s active, you don’t need to do anything special to benefit from it. Just do whatever you would normally do to earn XP. While Supercharged XP is active, you’ll earn a lot more XP for doing everything.