What is the role of voltage?

The function of voltage is to generate current in a section of circuit, there must be directional movement of charges, voltage can cause directional movement of charges, and the power supply is a device that provides voltage. 

Voltage is a physical quantity that measures the energy difference produced by a unit charge in an electrostatic field due to different potentials.

If the magnitude and direction of the voltage do not change with time, it is called a steady voltage or a constant voltage, referred to as a DC voltage, and is represented by a capital letter U. If the magnitude and direction of voltage change with time, it is called fluctuating voltage. For circuit analysis, one of the most important variable voltages is sinusoidal AC voltage 

(referred to as AC voltage), whose magnitude and direction change periodically with time according to the sinusoidal law.