Unity already comes with a simple logging mechanism, but if your using Android, you won't see any log messages in the editor. To receive your log messages, it was necessary to use some external tool.

This package extends the Unity Editor with an additional window for Android log messages, which connects with your local Android SDK and provides you with its logcat mechanism.
Therefore you can now see each and every log message of your Android device, even those of other applications. Of course you can filter these messages to only receive logs from certain applications or with a specific log level.
Furthermore this package also provides you with the ability to receive those log messages wirelessly. Just connect your phone, click on the wifi icon and remove the wire!

For more informations have a look at the Documentation.