When we say "Robots" file we don't literally mean a robot. "Robots" is a term reserved for a file that resides in the root of your website and effectively tells search engines which files to look at. Or more accurately, which files NOT to look at.
For example, if you have a folder on your web called "Clients" and you do not want these pages to be indexed, or spidered, you can use the robots file effectively to hide these pages from prospective browsers.
The structure and content of this file is not discussed in any detail here as although it should be borne in mind it does not directly relate to optimising a page for a search engine's index.