Terminology for Table of Contents with links to sections

Sorry but I forget what this is called. I have a 68-page document, and the first page has a table of contents (I think this is the terminology) that allows me to click on an item (on that first page), and then it takes me directly to that section. The sections titles in my document that are linked to in that TOC are all formatted as Heading 1.

I'm trying to add a new item to that TOC, but I can't seem to be able to do it, or to be find the info online. Is what I have, in fact, a TOC? Or is it something else? And also, how do I add an additional section to this TOC, allowing me to click on the link, that takes me straight to that section in the document?

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