Your battery needs to be charged. The radio is too low to run because it will consume the battery. The device is basically functional except with no service, like a dummy phone.
As defined by Merriam-Webster, a radio is "the wireless transmission and reception of electric impulses or signals by means of electromagnetic waves." In other words, it is a device that is used to send and receive messages.
In the alert message, it indicates that the phone can't communicate with its radio (antenna). This is either because the radio is turned off, or because there is not enough power to power up the radio. Your phone cuts power to the radio at around 10% to 15% battery power so that it can stay powered on longer (I think emergency phone calls can still be made up until the phone dies).
