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Three phase is a phrase commonly used to refer to the common mode of electric power delivery in the United States.

At the point of destination a substation or transformer supplies the power signal stepped down from the high voltage transmission values to 3 sinusoidally varying electromagnetic fields of 120 Volts Alternating Current (VAC). This is then delivered to the customer's circuits at master breaker panel via 4 conductors. One conductor is the neutral or ground at the power source, the other three lines act as waveguides for the electromagnetic fields delivering electrical power to point destinations or supply transformers. The electromagnetic fields are commonly alternating current (AC) which vary sinusoidally in time. Each of the three lines are 120 degrees out of phase with the other two lines. Connecting electrical circuits from 1 line to the neutral supplies 120 Volts Alternating Current (VAC) to the circuit.

Connecting between two power supply lines provides 240 VAC because the out of phase sinusoidal forms add to provide a 240 peak voltage in the resulting waveform. A common 60 amp service in older homes in the United States typically uses only 2 phases. Small load circuits are connected to breakers accross the neutral and one phase (pole). Large load items such as ovens, water heaters, electric furnaces are often connected across two phases (240 VAC, 220 VAC) for greater efficiency.

All three phases are typically used in large industrial motors as this provides the most efficient use of the electric power when starting the motor.

For further information on three phase circuits see:

How to test three-phase electrical supply
How to test three-phase pumps