Jetpack is a suite of libraries to help developers follow best practices, reduce boilerplate code, and write code that works consistently across Android versions and devices so that developers can focus on the code they care about.
AndroidX is the open-source project that the Android team uses to develop, test, package, version and release libraries within Jetpack.
AndroidX also is a major improvement to the original Android Support Library, that provides backport to older version of android platform. For example, LinearLayout class has setPaddingRelative() from API 17. If you want to use this method on lower API level, you must use the LinearLayoutCompat class instead, that has all new methods.
It is recommended to use AndroidX in all new projects instead android support library.
See list of all AndroidX artefacts.
migration to AndroidX
AndroidX replaces the original support library APIs with packages in the androidx namespace. Only the package and Maven artifact names changed; class, method, and field names did not change.
With Android Studio 3.2 and higher, you can migrate an existing project to AndroidX by selecting Refactor > Migrate to AndroidX from the menu bar.
After you will see in the gradle.properties file
The Android plugin automatically migrates existing third-party libraries to use AndroidX by rewriting their binaries.