When active, the XP gained for leveling up is multiplied by 5.

Q: What if I also have a 2X XP?
A: The 5X XP will come first, putting the 2X XP on hold.
Q: What if I've got two 5X XP boosts?
A: The durations of the boosts will add up.
Q: What if I also have the level-up rocket (shown in the picture below)?
A: The rocket will come first.