I think his question is more fundamental. Most digital signals are ordinary electrical signals that quickly (but not instantaneously) change between two voltages levels. At the receiving device, any voltage higher than a certain threshold voltage is interpreted as "one", anything lower is "zero". That's a simplification -- there are other details such as noise margin, hysteresis, rise time, differential mode, optical instead of electrical, etc.
What does DLD mean? Digital logic device maybe?