What are $DC$ signals and $AC$ signals ? Why do we preferred an $AC$ signal ?

$DC$ signals (current or voltage) do not change direction with time. They are unidirectional (current or voltage) signal.

If the voltage obtained from source varies like a sine function with time then such a voltage is called alternating voltage ($ac$ voltage).

Voltage - common used means potential difference between two points.

The current driven by in a circuit by ac voltage is called the alternating current ($ac $current).

The most of the electrical energy sold by power companies is transmitted and distributed as alternating current. The main reason for preferring use of ac voltage over dc voltage.

Second reason is that ac voltages can be easily and efficiently converted from one voltage to the other by means of transformers.

Further, electric energy can also be transmitted economically over long distances.