Beta software is software not ready for public release but is ready for testing. You can join the Apple Beta programme free by going to beta.apple.com.
Testing Beta software is not for everyone as it can lead to problems if something doesn’t work so the advice is to use a secondary machine for the testing.
Having said that, I use my only Mac and iPhone and iPad to run Beta software. So far it has been a good experience.
When you go to beta.apple.com to join the Beta programme it downloads software onto your device whether it be a Mac or iOS device. Once this has happened Software Update downloads the latest Beta version – which is Catalina 10.15.6 at this time of writing and soon to be macOS 11 Big Sur. On iOS and iPadOS it will soon download version 14. It also downloads Feedback Assistant which allows you to report problems back to Apple.
So on my Mac it downloaded 10.15.1 Beta and I wondered why it would not download later releases as it is expected to. After some research, I discovered that if you go to the Mac App Store, and get Catalina from there it will download version 10.15.5 which is NOT Beta. Once I did that it was able to upgrade to Beta 10.15.6 which is the latest Beta version. Clearly I could not get 10.15.6 until I had 10.15.5 installed.
I’m really looking forward to the public beta of macOS 11 Big Sur which will come this month. Stay tuned for reports on these betas once they are released.