For those of you who are confused this IS NOT AN APP. This is a feature of your Samsung phone that resides in the notification menu across the top
Wi-Fi, Bluetooth, Location, Power Saving, Sound, Sinc, Screen Mirror....
To enable it swipe down the notification screen > Find SCREEN MIRRORING > turn it on
It will look for a device to connect to. If you have the Samsung Allshare Dongle that connects to your TV it should find it. You may also connect to other devices.