The national contact tracing service has started today (28 May), guidance and info is available here:
The app is free and simple to use. Once you’ve downloaded the app, Bluetooth technology on your phone will record the distance between other phones that also have the app installed. If you become unwell with symptoms of coronavirus, you can allow the app to inform the NHS. This will trigger a notification that the NHS will then send anonymously to all other app users who you’ve been in significant contact with over the previous few days. Affected app users will be sent official NHS advice on what to do next.