HTML tags which help outline a web page or draw attention to important information are called Header Tags. Keyword inside Header Tags can boost rankings on the search engines. For Example

the HTML tagging that displays the text inside the tags in a size relative to the header tag number. H1 ... /H1 is the largest; H6 ... /H6 is the smallest. Text within header tags is bolded. The header tags also insert implicit paragraph tags before and after the header text; this sets the header text apart from the surrounding text or images. This is an H2 header

HTML tags that define your page's organizational structure and lay out page navigation.