Three-phase electric power utilizes three conductors that each carry an alternating current, meaning the current first flows in one direction in a circuit and then reverses to flow in the other direction, of the identical voltage amplitude and frequency relative to a common reference. The common reference that power companies usually use is the earth, which serves as one of the wires in the power system. Three-phase motors are popular because they provide consistent voltage.
In a one-phase or two-phase motor, there are many moments in each second when a sine wave is crossing zero volts. In three-phase power, one of the three phases is nearing a peak at any given moment. While some common electrical appliances may not need the consistent voltage of a three-phase power, it is much more important when powering essential, heavy machinery.
Power companies use the earth as one of the wires in a three-phase power system because the earth is a very good conductor and it is quite large, so it makes a reliable return path for electrons. In addition to being more reliable, a three-phase system is typically considered more economical than a similar one-phase or two-phase system because it uses less conductor material to transmit electrical power.