robots.txt

A robots.txt file is a file at the root of your site that indicates those parts of your site you don’t want accessed by search engine crawlers. The file uses the Robots Exclusion Standard, which is a protocol with a small set of commands that can be used to indicate access to your site by section and by specific kinds of web crawlers (such as mobile crawlers vs desktop crawlers).

For a full explanation of how to use the robots.txt file, read the documentation provided by Google.