Facebook Pixel (now Meta Pixel) is a JavaScript snippet installed on your site that tracks visitor actions: page views, add-to-carts, purchases, form completions. The data is used to optimize Meta Ads campaigns, build custom and lookalike audiences, and run remarketing.
With Facebook Pixel installed, you can build a remarketing audience of people who added products to their cart but did not buy, then show them an ad with a 10% discount code.