A heading is a title or a subtitle displayed in a document or webpage. Headings are created with the <h1> to <h6> HTML tags where <h1> is the main heading and <h6> is the lowest level heading.
Headings in a document serve a dual purpose: they help the creators and readers interact with the content and affect the way search engines index the webpage.
Having a clearly defined structure with sections and subsections is beneficial for both the creator and the readers. The creator can convey their message better and emphasize important parts. A few informative headings can help the readers scan the document in search for specific information.
Search engines use the headings to index the structure of the document and point to important keywords.