A Title tag is the title of a webpage as displayed in the top of the browser window and the blue link in the search results above the meta description. This differs from the on-page title, or header, of the content.
The <title> tag is held within the <head> tags of a tags at the top of an HTML webpage. It is on of the most important elements of the page.
<title>The title of your page goes here</title>
The search engines place greater importance on the copy in the title tag than on any other part of the page. Even though the user is often oblivious to this element, it's of great importance that you optimise your title tags for search by using your target keywords and a succinct description of the content of the page.
Title tags are tricky because as well as optimising for search engines, as explained above, you also have to optimise for users who encounter the page as an option in the search results. Interesting title tags that match the user's intent are more likely to recieve a higher click-through rate.