Let's take an example.org document, the simplest example of a an org-mode buffer with just a few headlines:

* Headline 1 This is some text * Headline 2 This is heading number two * * Sub heading 1 * * Sub heading 2 * Headline 3 In conclusion, this is an example

Now, if you invoke C-c C-e to bring up the export options and export this document to HTML, you'll end up with a document that looks something like this:

Currently this is at the file:///home/hinmanm/example.html url. In the table of contents, if you click on the Headline 2 heading, you are then taken to the file:///home/hinmanm/example.html#sec-2 and jump to that location on the page (imagine the page is longer so you would jump to the location).

The issue with this is when we edit our example.org file to add a headline somewhere in the middle, in this case, adding a "Newly added headline" in between "Headline 1" and "Headline 2".

* Headline 1 This is some text * Newly added headline Wait, I've decided that this information is really important! * Headline 2 This is heading number two * * Sub heading 1 * * Sub heading 2 * Headline 3 In conclusion, this is an example

If this is again exported to HTML, we get: