Header tags are HTML tags used for styling a site and also used by search bots to rank the content on the page.
Text within header tags has a greater weighting than ordinary body text content. Headers are typically marked <h1>, <h2>, through to <h6>. With <h1> getting the higher weighting and <h5> getting the lowest.
The h1 tag is an integral part of the web structure and can be likened to a newspaper or magazine headline or book chapter title. Consequently, search engines consider the text between the h1 tags to be particularly important as this text is expected to describes what the page is all about and rank these words higher than other words on the page.
There should always be a topic for any page and consequently there should be an h1 tag that contains the appropriate words for this topic. Any sub-headings should use the lower order header tags (h2,h3...). As a general rule have only one h1 tag and then cascade the other lower order tags (h2,h3…) below this.