Lookalike Audiences are a way from to reach new people who are likely to be interested in your business because they're similar to customers you care about.

You can base your Lookalike Audience based on a custom audience, which means you'll be able to target people who Facebook believes are similar to the people inside your custom audience.

You can use ConnectRetarget-audiences to create lookalike audiences as well.

Once you've created an audience using ConnectRetarget, you can create such a lookalike audience inside your Facebook Ads account.

You'll find instructions how to create a lookalike audience here.