A beta reader is, essentially, someone who reads your work and offers input while it is in draft form. Generally, they look for typos, grammatical errors, continuity issues, etc. in order to help improve and polish your work before its submitted to a publishing professional or made public. (from the website of Literary Rambles) There are more definitions out there but this about sums it up.
Need some more great information about beta readers and what all the fuss is about? You can read another informative article about beta readers on Small Blue Dog Publishing. Actually there is no fuss. All authors need beta readers and from what I can understand from the little bit I have read is they are going to be a lot more helpful than Sister Sara, Aunt Sue or Mom when it comes down to the wire! Anyone you are related to or close to tend to be a little prejudice toward your writing. Don’t you think?
I could research and copy and paste websites for you all day, but I gave you a place to start and now I’m going to go read up some more about beta readers because I am going to need one soon for ‘What Line’! Maybe I’ll see you there. ;)