A Title Tag is meta information inserted at the top of a web page's source code, and displayed in the browser bar. Search engines also use the title tag to provide a link to the sites witch match the query made by the user.
An example of title tag code in the context of a complete HTML document:
<title>eCommerce solution trusted by more online merchants! Shopping Cart Software by ezimerchant.</title>
Title tags are probably regarded as the most important piece of content that search engines look at when determining the ranking of your site. Be sure to set appropriate title tags for each of your pages. Don't worry though, with ezimerchant you don't need to know HTML, it is taken care of for you.