Robots.txt is a file where you can specify what webcrawlers can/can't do on your webpages. Preventing them from indexing your site/app.
Heres's some more information on it. http://www.robotstxt.org/
There are some older questions related to robots.txt on the forum. Where it states that robot.txt can be, or should be found in the Theme. But a quick search in my own projects didn't show any robots.txt file.