Heading tags are used in HTML to create structure. They mark titles and subtitles or emphasized text. There are six levels of headings in HTML – H1, H2, H3, H4, H5, and H6. The higher the level, the more important the heading.
They are the simplest way of telling both our reader and the spiders what we believe is the importance hierarchy of the content. Do not use them to define font size, it is better to do this through CSS. You should always keep to the order of the headings, do not use an H4 unless you have an H2 and an H3 also.
Assigning an H1 to a title means that this is the most important element on your page and you should only use one.