A beta reader is someone whom an author sends writing to be critiqued. The basis for doing this is pretty simple: someone else can look at the writing with fresh eyes and pick up things that the writer might miss, and it can be useful to have another perspective.
Very many authors, including top writers, make use of beta readers. I don't (or, at least, not yet), but might change my mind in the future. The potential downside is that a beta reader has to be critical enough to pick up mistakes, from straightforward typos to problems with tone or continuity, without being hyper-critical. There's also the need for the author to be able to take criticism and to have confidence in their beta reader.
Note that a beta reader is not an editor or proof-reader, although their suggestions and comments might include thoughts on editing and the spotting of typos. It's not a paid job, it's a helping hand from someone who loves to read for someone who wants some (hopefully objective) feedback. Authors, perhaps obviously, sometimes do it for one another.
I recently did some beta reading for someone (for the first time), and found it quite a refreshing experience. It probably helped that the writing was easy to read and good, but it was far more relaxing and enjoyable than critiquing my own stuff (I managed to read 16,000 words on the day I got the file and sent back my thoughts the following day). The hardest problem I faced was that when critiquing my own writing I'm relentlessly negative (because good stuff can be left as is whereas bad stuff must be identified and changed/deleted) and it was a bit odd trying to remember to say nice things as well.
So, if you're a new author why not consider beta readers? At worst you can ignore silly suggestions, and at best they can offer great insight.