A flat XP season is a season in Fortnite where every level is 80,000 XP and no more above. The last non-flat XP season was introduced in Chapter 3, and is a season where the XP consecutively increases each level and becomes harder to level up.
A flat XP season is not very common to see, as Fortnite mainly only adds flat XP seasons on mainly every season.
Flat XP: I need 80,000 XP to level up
Non-flat XP: I need 1,600,000 XP to level up (usually at level higher than 200)