Our text represents a wave moving to the right as y = A sin(k x - ω t + φ0). It is very convenient to represent the wave by using a phasor.
A phasor is a line on a complex plane that represents a wave at the location x = 0. The length of the phasor equals the amplitude of the wave--in meters for waves on a string, pascal for sound, V/m for light, etc.
The initial angle of the phasor is the phase constant.
The phasor rotates clockwise with the frequency of the wave. In our text (Halliday Resnick Walker) the wave is interpreted as the projection of the phasor along the y-projection.
Other texts may use the projection along the x-axis (cosine instead of sine) and may use a counterclockwise rotation.
What is the frequency of the wave represented by this phasor?
What is the amplitude of the wave represented by this phasor?
What is the phase constant of the wave represented by this phasor?