Glossary Term: H1, H2, H3
H1, H2, H3 tags indicate the importance of each header. H1 tag indicates the most important header. Like title, Usually it represents the whole document. H2 and H3 tags represent documents that are less important. Usually it is the sub-content from the whole document. Although there' h4, h5 and h6 tags as well. But those are not as important when performing Search Engine Optimization (SEO).
Keywords wrap inside H1, H2, H3 tags receive higher keyword importance score when search engines is crawling the page. It is a good pratice to have at least one H1 tag on each page to give search engines a better idea what is your page content is about. Also, font size for H tags are usually bigger than regular tags. For human readers, it is much easier to navigate through the page to look for the information they need.