An H1 is a page’s main heading — the large, prominent line a visitor actually reads at the top of the content. It is a different thing from the page title: the title is what shows in a browser tab and search result headline, while the H1 is what appears on the page itself.
Two ways this goes wrong
A page can have no H1 at all, leaving both visitors and search engines without a clear statement of what the page is about. Less commonly, a page ends up with more than one H1, which muddies which heading is actually the main one.
A missing H1 and a missing page title are easy to confuse, but they serve different purposes and are checked separately.