What does "in beta" mean? I am an uncultured swine so I am not sure that I understand what that term means. Can someone please explain?
It means the tree isn't fully developed by the Duolingo curators and is still in the process of being expanded and checked for flaws.
It's a general term that you will also encounter outside of Duolingo, especially referring to software. When software (or sometimes other products) are being developed, at some point they will begin to test how it's doing and if it's ready for distribution to the public. The first level of testing is called "alpha testing" (named after the first letter of the Greek alphabet) and is usually done in-house or a firm is hired to do it. The second phase of testing is to do a limited release to members of the public. This is called "beta testing" (beta is the second letter of the Greek alphabet). This allows the testing pool to be much larger and be more likely to find the problems that the programmers never anticipated. I suppose it's possible that some companies move to "gamma testing", but usually everything stays in "beta" until they are fairly confident that they've found and corrected all the problems. So "beta" usually refers to an early public release that is not really to be considered complete or final.