The Facebook pixel is a snippet of code that you can place on your website. It will collect data with which you can measure conversions from your Facebook ads, optimize them, create your target audience for future campaigns and retarget users who have already performed an action on your website.
It works by activating cookies that have the task of monitoring user interactions on your website and in your Facebook ads.
In the past, there were two different types of Facebook pixels: conversion and custom audience.
The former was abandoned by Facebook in 2017. Even if you used it in the past, now you have to switch to the new one. To learn how, read this Facebook business support article. If you were using the old custom audience pixel instead, these instructions tell you how to upgrade to the current pixel.