Apple Support, channeling the dictionary:
Beta is a term used to describe software that is in the last stage of testing. Software publishers frequently make beta software availabile [sic] to a limited audience so that it can be tested in real-world environments. This is called a beta test.
Because beta software may be incomplete, it may not function correctly all of the time. You should only install beta software if you know how to remove it, or how to troubleshoot errors it may cause. Before installing any beta software, be sure you have a backup of important files you want to keep.