robots.txt is simply a text file that you place in the root of your web folder that gives instructions to spiders such as Googles Googlebot - [google.com...] or Yahoos Slurp - [help.yahoo.com...] what it can and can't crawl. By default if you don't have a robots.txt it assumes everything on your site is freely crawable, if it is you don't have the need for a robots file. If however you don't want it crawling a certain page you can indicate this in the file.
This page has more details (hope it is ok to list, sure i have seen it here before)
[robotstxt.org...]
But to answer your quesion, the lack of a robots.txt file won't be hurting your rankings.
