“Google Tag Manager” allows you to wrap several different codes through one “Tag Manager” Code. It’s really cool, you can have all your Google Analytics, Google ads and pretty much any “3rd party code” wrapped in one “tag code”. That way when a client (or you) want to update any tracking code, they don’t need to touch the website at all, just update their “Tag Manager Dashboard”
There should be 2 pieces to the code - a “script” code that goes into the “head” of each page, and a “noscript” code that goes into the “body” of each page