The H1 is the primary headline on the page itself. It should describe the topic clearly and match what the visitor expects after clicking a result. Most pages should have one, and the rest of the headings (H2, H3 and so on) divide the page into sections beneath it.
It is related to, but not the same as, the title tag. The title tag is what shows in search results and browser tabs; the H1 is what readers see on the page. They can be worded differently but should clearly agree on the subject.
Headings also help people scan and help search engines and AI tools pull out the structure of a page, which supports AI-Readable content. Avoid using heading tags just to style text, and keep the hierarchy in order.
Related: Search Engine Optimization, Meta Description, Technical SEO.