The robots.txt file (Wikipedia page) is read by search engine crawlers to know which parts of your website should be crawled.
It’s important to realize that your website can still be listed in Google even if it is not crawled.
This usually happens when other sites link to yours.
If you would like to completely block your website from being listed, it’s actually important to let it be crawled, but to include
noindex tags in each page.
More information is available on the Google page about
Name: anything you like
File contents: your site is set up with two basic
robots.txt files — one that allows crawling and one that does not.
If you would like to exclude certain parts of your website, you can easily create a new
robots.txt page to do so. See the Google page for details.