It fetches the page and builds the same heading outline a screen reader or Google's content parser would see, then checks two specific things: whether there is exactly one H1 on the page, and whether every heading level increases by one step at a time rather than jumping (an H2 followed directly by an H4, for example).
How to fix what it finds
If you have zero or multiple H1s, pick the single heading that best represents the page's main topic and make that the only H1 — demote the rest to H2 or lower. For skipped levels, insert the missing intermediate heading, or if a section genuinely doesn't need its own heading, remove the deeper one rather than leaving a gap in the sequence.