The opening tag alerts a browser that this is the start of your HTML document.
The opening tag contains your header information and is not visible when you view the page in your browser. Any text displayed between the BODY tags ( and ) will be displayed in your browser.
The closing tag alerts your browser that this is the end of your HTML document.
For ads to appear properly, you need to place your ad code between the opening and closing tags of your source code.
If you're having trouble finding the tags, try pressing the 'CTRL' and 'F' keys on a PC, or 'COMMAND' and 'F' keys on a Mac to open a Find text box. Then type ' tags is important, because you need to paste the AdSense ad code in between the and the tags of the document.