HTMl, or Hyper Text Markup Language is the language of web pages. It is composed of "tags". Tags are simply a set of predefined markers, or indicators. These are read by browsers and search engines and are used to determine context, importance and appearence.
Many web pages do not make effective use of H1 Tags. The H1 Tag is simply <h1> your text here </h1>. Notice how we clsoed the <h1> tag with the / - making </h1>. HTML tags must always be close and as such are usually found in pairs.
H1 tags are often used by search engines to determine the importance of a page, and the topic it is about. Ideally, your H1 tag should match or closely follow your meta tags, page title, filename and main keywords.