These are standardized headings that a page can have. Think of them like an outline that would define the structure of the content. We recommend the first headline on a page should be a Heading 1, or H1. H2 might define sub-headings of content down the rest of the page. If you needed even another tier or organization, you might use an H3. One H1 and multiple H2's usually work well.
Structuring the content of your page using headings not only helps Google prioritize your content, but it helps the casual visitor as well.