When exactly does Fey Ancestry apply to saves?

@JeremyECrawford When exactly does Fey Ancestry apply to saves? Just to avoid the "charmed" condition?

— Matthew Domville (@mattdomville) November 25, 2016

Fey Ancestry applies when an effect attempts to impose the charmed condition on you. "Charmed" means "charmed condition" in the rules. #DnD https://t.co/kXueS4jaKi

— Jeremy Crawford (@JeremyECrawford) November 25, 2016