In a few words: it's like Sketch Mirror for Android.
Mira lets you real-time preview your Sketch designs on your Android devices.
You don't need to worry about the different screen sizes and resolutions, Mira will auto scale them automatically and deliver pixel perfect results.
You can also connect as many devices as you want over the Wi-Fi network. We are currently working hard to allow people connect via USB, so be patient we are getting there :)