It is an alarm sounded, which is a Government test hoped to use in the future if there is a threat to life, on compatible phones and tablets. You do not need to do anything.
You will also not receive an alert if:
- Your phone is turned off or in airplane mode
- You’re connected to a 2G or 3G network
- You’re using Wi-Fi only (so not connected to a network)
- Only phones and tablets running iOS 14.5/Android 11 or later will receive the alert
Be aware that the government advises keeping the alerts on for your own safety.