You are looking at a Web page right now. The code on the left creates the browser display on the right. In HTML, a tag tells the browser what to do. The tags you write are not visible on the browser, but their effects are.
Tags usually come in pairs, one that begins an action and one that ends it.
When you write an HTML page, you see all the tags and can change them. When you view the page on a browser, you see the result of the tags. Most programs that help you write HTML code already know most tags, and create them when you press a button. But you should understand what a tag is, and how it works. That way you can correct errors in your page more easily.