HTML tags that are used to categorise text headings on a web page.
There are 6 types of header tags (H1 to H6), varying in size & importance. A page should only have one H1 heading (title). But the H1 heading can have multiple H2 headings (sub-headings) underneath it, H2 headings can have multiple H3 headings (sub-sub-headings) & so on.
These help create a visual hierarchy, so website visitors & search engines understand the structure & content of the page.