So, for example, run an ad to an article about a widget you sell. The article discusses the benefit of the widget and how to use it. Then the pixel records who visited the article from the ad. Then you make an audience of people that only visited that article. Then you run a separate traffic campaign to only that audience, but this time your ad is for the widget you're selling. When you have enough traffic that has come through and purchased the item, you have enough data for Facebook to use conversions instead of or in addition to traffic.