Adding to Tom's answer:
The wording is not very clear, but what this means is that digital signals do not actually exist in reality. All signals are analog.
When we decide that a voltage above a certain threshold is a "1", a voltage below a certain threshold is a "0", and the space in between is "undefined", then we interpret an analog signal as a digital value. However, it is only a very convenient approximation that greatly simplifies the job of the designer.
Digital is abstract information. It is a meaning we choose to assign to physical values. This is why you cannot send a digital signal over the air as radio waves. It must be converted first into something that exists outside of abstraction, like an analog signal which represents the information to be transmitted.
The real signal is made of physical analog values: voltage, light, current, fields, acoustic pressure, whatever.
For your radio application, you could encode your digital bits into the frequency of a carrier, or its phase, or any other encoding, of which they are many. Now, you have an analog signal which carries your information, and you can transmit it, then receive it and recover your bits.