AndroidX is the open-source project that the Android team uses to develop, test, package, version and release libraries within Jetpack.
AndroidX is a major improvement to the original Android Support Library. Like the Support Library, AndroidX ships separately from the Android OS and provides backwards-compatibility across Android releases. AndroidX fully replaces the Support Library by providing feature parity and new libraries. In addition AndroidX includes the following features:
All packages in AndroidX live in a consistent namespace starting with the string androidx. The Support Library packages have been mapped into corresponding androidx.* packages. For a full mapping of all the old classes and build artifacts to the new ones, see the Package Refactoring page.
Unlike the Support Library, AndroidX packages are separately maintained and updated. The androidxpackages use strict Semantic Versioning starting with version 1.0.0. You can update AndroidX libraries in your project independently.
All new Support Library development will occur in the AndroidX library. This includes maintenance of the original Support Library artifacts and introduction of new Jetpack components.
Source : AndroidX Overview