What is Facebook Pixel?
In simple terms, Facebook Pixel allows you to put a tracker on your website visitors. To do so, you'll need to install a piece of code into your website.
When your visitors visit your website, they will be gathered as a custom audience in Facebook Ads Manager.
You can serve retargeting ads to these visitors via Facebook & Instagram. As these visitors visited your website before, they will be more likely to visit your website or finish the incompleted transactions before. The ROI will be much higher.
the Facebook Pixel: https://www.facebook.com/business/learn/facebook-ads-pixel