Phase difference

Two ac waves might have exactly the same frequency, but they can still have different effects because they are “out of sync” with each other. This is especially true when ac waves are added together to produce a third, or composite, signal.
If two ac waves have the same frequency and the same magnitude, but differ in phase by 180 degrees (a half cycle), they will cancel each other out, and the net signal will be zero. If the two waves are in phase, the resulting signal will have the same frequency, but twice the amplitude of either signal alone.
If two ac waves have the same frequency but different magnitudes, and differ in phase by 180 degrees, the resulting composite signal will have the same frequency as the originals, and a magnitude equal to the difference between the two. If two such waves are exactly in phase, the composite will have the same frequency as the originals, and a magnitude equal to the sum of the two.
If the waves have the same frequency but differ in phase by some odd amount such as 75 degrees or 310 degrees, the resulting signal will have the same frequency, but will not have the same waveshape as either of the original signals. The variety of such cases is infinite.
Household utility current, as you get it from wall outlets, consists of a 60-Hz sine wave with just one phase component. But the energy is transmitted over long distances in three phases, each differing by 120 degrees or 1/3 cycle. This is what is meant by three-phase ac. Each of the three ac waves carries 1/3 of the total power in a utility transmission line.