HTML (Hyper Text Markup Language) is a formalised set of rules used to display content on a web browser, like Internet Explorer or Firefox. The basic building blocks of HTML are called tags. Each unique tag has an operation to perform, such as displaying an image, or highlighting some text, etc. Learning HTML is beyond the scope of this article, so it is assumed for this section that you have basic HTML knowledge.
One of the important aspects of SEO (Search Engine Optimization) is performing what is called "On-page SEO". This basically means taking the HTML tags and associated content and manipulating them to more closely resemble what the Search Engines (SE's) would rank higher in their Search Engine Results Pages (SERP's). To do so means gaining access to the server on which your site is hosted and tweaking to a degree the HTML tags. If all of these sounds like it is a bit too technical, then contact your webmaster and ask him/her to do it for you.