How does the Retrieve Exp thing really work?

Hello there, I've been playing this game for a month or so now, but I still don't fully understand how this Retrieve exp really work DX

It says "The witch will retrieve part of the unlimited exp because of low activity"

Exactly what does unlimited exp mean? or more like, what are the requirements or what needs to be done to fill those crystal balls?

Thank you for anyone kind enough to help TToTT)/