The Facebook Pixel was launched at the end of 2015 to combine the functionalities of the old Conversion Tracking Pixel and Custom Audience Pixel (which will no longer work from February 2017) and also provide some new and improved functionality.
The Pixel is a snippet of Javascript that you add to your website's code. But it's not as complicated as it sounds. At a basic level it tracks every page view on your website by a Facebook user. This allows you to:
(i) Track specific actions and conversions on your website.
(ii) Build custom audience lists to run campaigns retargeting users of your website.
Source: https://www.adgo.io/blog/2016/12/22/how-to-install-the-facebook-pixel-a-step-by-step-guide