- A business creates an ad. Let's say a gym opens in your neighborhood. The owner creates an ad to get people to come in for a free workout.
- Facebook gets paid to deliver the ad. The owner sends the ad to Facebook and describes who should see it: people who live nearby and like running.
- The right people see the ad. Facebook only shows you the ad if you live in town and like to run. That's how advertisers reach you without knowing who you are.
Reference: Facebook Advertising