where to post code

Hey all!
I am needing to post code and I’m a little unsure how to do this.

“In the HTML of all the pages in your website, place the attached code just before the </body> tag and any other measurement tags.”

I am unsure where to post this. Do I just have to post it in the body code in the customize part before </body> or do I have to do something different? Also measurement tags? What else am I looking for?

Thank you very much!