In Kingmaker, you have 2 characters: your Active Character and your Background Character. Normally, your Background Character is earning XP while defending the home base and going on their own adventures “off screen.” But what happens if you decide to take your Background Character out adventuring?
Then the 2 characters switch roles. That character is now your new Active Character.
Your new Active Character also is behind the rest of the group in XP. So long as this character is behind the rest of the group, the character gets a +50% XP BONUS until he or she has caught up.
Meanwhile, the character that is now staying behind is now earning XP at a slower rate: the amount of XP it would earn if it were one level behind the main adventuring group (i.e., as much as the other Background Characters are earning).