Tags are used to communicate information to the browser about how we want text, images, and shapes to show up on the page. If you look at the code behind this page, you can see an open tag like this: <html> at the top of the page. At the bottom of the page, you can also see the closing tag </head>. Most tags follow this format, with an open and close tag surrounding the text and code it is addressing.
The <body> tag contains all of the visible text in the body of the page, and the <head> tag includes information like the title, the style of the text and, if needed, additional scripts and information. Many sites use the head tag to include script for google analytics or other services to track users visiting their site.
You may also notice the <br> tag, which doesn't require a closing tag. That's because "br" creates a line break, and doesn't need any additional information or text that it's modifying. These activities will require you to modify and add tags to change the way the pages look.
If you get stuck or want some more info on writing HTML, W3 Schools has a reference page for HTML tags.