A typical digital signal is a voltage that alternates between (typically) 0 volts and +5 volts, with varying durations. In some cases , if the value is not varying, it may be one wire, or a set of wires, each with either zero volts or +5 volts on them, not changing.
Actual voltages are between 0 and 0.4 volts (a low or zero) and between 2.4 volts and 5 volts (a high or one). This is for TTL. There are other logic families that have different voltages.
A typical time varying digital signal such as a serial data stream will consist of a set of pulses with a fixed period, but with a pattern of highs and lows that hold the data. To a casual eye, the pattern seems random.
But you won't see a digital signal with a fixed unvarying duty cycle as you would see in a square wave.
Now a square wave is fixed. The period is fixed, and the high to low ratio is fixed. Typical square waves are bipolar, that is, the low is, for example, –10 volts with the high at +10 volts. Unipolar square wave would have a low at zero volts and a high at +10 volts. The pattern is fixed with a fixed duty cycle.