Just like in Word processed documents, headings play an important role in the structure of your web page. Users can see what the headings are for your document and can skim your pages according to its headings. Search engines use the headings to index the structure of the web page. In HTML, there are six levels of headings.
The h1 heading should be used for main headings, h2 is used for subheadings, and if there are further sections under the subheadings, then you can use h3 up to h6.