With the Android M SDK (API 23), Google removed the Apache HTTP Client library. It was deprecated since API 22, and Google recommanded to use
HttpURLConnection instead since API 9. While the classes are still bundled in Android 6 ROMs, it won’t be long until we see them completely go away.
Some applications are still relying on this library, and need to be updated to use the SDK 23, without having time/budget/whatever required to switch from HTTP Client. While I strongly recommend you to still take time to move to something else (there are many high-level libraries, like OkHttp or Ion, or you can use
HttpURLConnection to keep a low-level access), there is a way to use the Apache library while using the SDK 23.