Pre-releases are available for the desktop application. They are pretty much like regular releases, except that they have not yet been tested by many users, so it is possible that a bug or two went through.
You can help the development of Joplin by choosing to receive these early releases when updating the application. If you find any bug or other issue, please report it on GitHub or the forum (if you do not have a GitHub account). Any bug report during the pre-release phase is invaluable to us, as it allows making the app more reliable before making it available to more people.
In general it is safe to use these pre-releases as they do not include any experimental or unstable features. Hundreds of users run them without any issue. Moreover even if you do find an issue, you can report it and it is usually fixed very quickly as these bugs are given the highest priority.
To have access to these pre-releases, simply go to Configuration > Application and tick the box "Get pre-releases when checking for updates".