What is a Facebook Pixel?
"A Facebook pixel is code that you place on your website. It helps you track conversions from Facebook ads, optimize ads based on collected data, build targeted audiences for future ads, and remarket to qualified leads—people who have already taken some kind of action on your website. It works by placing and triggering cookies to track users as they interact with your website and your Facebook ads."
Read More: https://blog.hootsuite.com/facebook-pixel/