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.

In order to make a robots.txt file, you need access to the root of your domain. If you're unsure about how to access the root, you can contact your web hosting service provider.

Please find more information about the Robot.txt file here:
​https://support.google.com/webmasters/answer/6062608?hl=nl 

And information about how to create the file here:
​https://support.google.com/webmasters/answer/6062596?hl=en&ref_topic=6061961