If a spell or ability would charm the character or creature with Fey Ancestry and if the spell or ability allows a saving throw to avoid the effects, the character or creature with Fey Ancestry gets advantage on that saving throw.
One example is the spell Crown of Madness, which reads:
One humanoid of your choice that you can see within range must succeed on a Wisdom saving throw or be Charmed by you for the duration.
Here's how it would work. Let's say we have an Elf character, because Elves have Fey Ancestry, and someone casts Crown of Madness on this Elf. The Elf has Advantage on the Wisdom Saving throw because the spell has a Charm effect.
Additionally, the Fey Ancestry feature reads (emphasis mine):
You have advantage on saving throws against being charmed, and magic can't put you to sleep.
If the spell or ability says it charms its targets, and if it offers a saving throw, the targets with Fey Ancestry get Advantage on such saving throws.
I hope this answer is to your satisfaction.