I can't answer about LDW in particular, but I've taken part in several beta (and some alpha) tests for several types of software in the past.

Basically the testers will download the beta version and play it, reporting any bugs, problems, and suggestions to the software makers. They will look at the reports, fix the bugs, add possible new features, and make another version available for testing. The testers will go through the same process, checking that previous bugs have been fixed properly, and that no new bugs have crept in. The developers will again make a new build available, receive more feedback, and so on until the game is as _bug_-free and playable as possible. That can involve only one or two builds or perhaps several. Some software I've tested is in an almost permanaent stage of beta testing.

Once they are sure the game is as good as it can be, then, and only then, it is released.

There will be several beta testers to get as wide a variety of types of machine and player as possible. I don't know how many, though.