A table of contents tells your readers what they can expect from an article, and provides instant jump-links so they can quickly find what they want.
Choose the following settings.
Now when you have headings set up in a page or post... (In this image you see h2 and h3)
... when you preview it you'll see a table of contents. Nice!
If you don't want 'My advice' to show in the table of contents, then you have to change it from h3 to normal text. You can BOLD it to make it stand out.
Another option is to change the advanced settings to exclude lower headings:
Now you can set things to heading 6 and it won't show in the TOC.
But personally I just use bold. It looks fine.
Why Hide It?
When you create a long piece of content the Table of Contents kind of looks ridiculous. So by default I hide my TOC - people can see that there is one and that already makes you seem professional.
Note that when you're logged in to Wordpress, the TOC will NOT hide itself from you. I have no idea why it does that, but it's actually useful for spotting mistakes in your headings.
When you use an incognito browser you will see what your visitors will see, and it will be hidden.