The Exposure Notification API was developed with Google and will only be activated if you decide to install a compatible Covid-19 tracing app from a health authority in your country. It means if you use the app to trace your movements in public spaces, the app and system will be able to send you a notification if someone alerts the authority they have tested positive for Covid-19 and you are logged as within their vicinity at a recent location. The idea then is you can be tested.
Apple and Google have worked fast to ensure the rollout of this API to as many phones worldwide as possible. It is said to be very secure and was developed to help authorities build apps that are most useful and informative to users.