Game preview video has to be very short: max. 30 seconds (one of App Store requirements)
People can watch the video on app stores before they download the game. Try to give sneak peeks of all your game activities. Show the best, most interesting and most beautiful parts of your game. Show your potential players how much fun they will have playing your game!
Before you start making game video, we suggest to watch a few examples on YouTube. It won’t take long!