robots.txt file - What Is It?

Simply put the robots.txt file tells what parts of your website that you want the web "spiders" (including adsense's web "spiders") and search engines to access. You put this file on your server - which means that you place it in the same online server that hosts your website(s).

As you can see from the name of the file it is a .txt file which you can create using the simple notepad program.

You can find more specific information about robots.txt file on Wikipedia

(more specific information was given to the person that asked this question for an additional price)