A Facebook Custom Audience lets you target your Facebook ads at a specific group of people who have already interacted with your brand.
So, instead of targeting your ads to people through luck or broad demographics like age or gender, you can target your ads based on data pulled from your own website, app, offline lists or from Facebook itself.
Using a Facebook Custom Audience allows you to do some pretty cool things, such as:
Retarget ads to people who have interacted with one of your Facebook posts.
Craft ads for specific segments of your audience, like your bottom-of-the-funnel email list.
Upsell new products to people who are currently customers.