On the Package Manager window, the indicator lets you know that there are preview packages currently installed in your project. By default, preview packages do not appear in the list of packages you can install; however, you can force the Package Manager to show them by enabling the Enable Preview Packages project setting.
Preview packages have not yet been verified to be safe to use with the current version of Unity, so you should only use them for testing and to provide feedback. At some point in the future, they might pass the verification requirements; however, they might also be deprecated instead. Because there is no guarantee for future support, you should not use any package that appears with the indicator in production. For more information about the preview state and the lifecycle of packages, see Package states and lifecycle.
For a list of stable packages verified for 2020.2, see Verified packages.
Click on any of the following links to see the documentation for the package.