Lookalike audiences helps advertisers to reach potential customers who share similar characteristics with their current ones. Both online and offline, lookalike targeting has worked well for businesses. This new option is available on Facebook Ads.
Lookalike audiences allow advertisers to target more people who look like their established customers. A lookalike audience uses either an existing custom audience or conversion data (conversion pixels) as a "seed" and an audience is built of similar users. Lookalike audiences can be used to support any business objective: Targeting people who are similar to sets of customers for fan acquisition, site registration, off-Facebook purchases, and coupon claims, or simply to drive awareness of a brand.
More about :
https://developers.facebook.com/docs/reference/ads-api/lookalike-audience-targeting/