Unity Remote, sometimes called Unity Remote 4, allows you to use your target device as a remote control for your project within Unity Editor. It's a huge time-saver since you no longer have to build and run on your device at each change you make.
The Game view is streamed to your device's screen, while the touch input, accelerometer, gyroscope, and device camera streams are streamed back to the editor.
Of course, you should still build and run regularly on your device to detect performance issues early and test your game with the real feel and reactivity.
Unity Remote requires that you connect your device to your computer via USB while it's running the Unity Remote 4 app. It works both for Android and iOS, on Windows or OSX. For iOS devices, the iTunes software must be installed.
First, you must download and install the Unity Remote 4 app on your device from the following links:
Android: http://goo.gl/Q7rZ19
iOS: http://goo.gl/U7eMCz