The RAC Black Box app is designed to run in the background but you don't need to open the app for this to work - it'll just run by itself.
It is a condition of your policy that the RAC Black Box app is running while any driver named on the policy is driving the vehicle.
You need to make sure that all drivers' phone settings allow the app to run in the background. Some battery-saving settings can stop the app automatically.
You don't need to open the app each time you drive and please don't use the app while you're actually driving!