What is digital signal ? What mathematical method are expressed in it?

Explanation

In electronic circuits such as amplifier, oscillator, the signals is in the form of constant change with current or voltage over time. Such signals are called continuous or analog signals.
A typical analog signal is shown in figure (a) and (b).

If a current or voltage (signal) has only two minimum and maximum values, the signal is called digital signal.
The signal in figure (c ) is in the form of waveform or pusle, which has only two values of the signals.

A binary system for displaying such signal is convenience. Binary only has two digits 0 and 1.
The maximum value (5V) of voltage is expressed as 1 and the minimum value (0V) is expressed as 0.
The input and output voltage indicated by two values (0 and 1) in the digital circuit are valid.