The Facebook pixel is a piece of code that enables the passing of information between Facebook/Instagram and your website.
This code allows you to track visits from Facebook/Instagram on your website, track campaigns and conversions. It also allows you to build remarketing audiences on Facebook/Instagram based on activity on your website.
There is only one pixel per Facebook Ads account, which is added through Google Tag Manager. Once installed, it does not need to be amended or changed unless the Ads Account does.