How to read the result
Headings are the table of contents of a page. The H1 says what the whole page is about; each H2 names one part of it; an H3 names a part of an H2, and so on. Search engines use that structure to understand which words matter and what each section answers, and screen readers let people jump from heading to heading. The outline on this page shows the headings exactly as they sit in the HTML, indented by level, so you can see whether the structure tells the same story as the design.
A skipped level is a jump such as H2 straight to H4. It usually means a heading was chosen for its size rather than its place in the outline. Use the level that fits the structure and change the size with CSS. An empty heading is one with no text: an icon, an image without alt text, or a heading tag used for spacing. It says nothing to anyone who cannot see the page.
One H1 per page is the safe choice. HTML allows more, and Google says it copes, but several H1s make it harder to say what the page is about in one line. If a page has two subjects that each need an H1, it is usually two pages.
What this tool does not judge
It reads structure, not words. A page can have a perfect outline and still have headings that nobody searches for. For that, write each heading as the question or the thing the section answers, in the words people use, and check the title separately.