The phase of a wave, measured in degrees, where 360 degrees is one wavelength, indicates the current position of the wave relative to a reference position. For example, if at time T1 the position of the wave along the vertical line was:
while at time T2, the position of the wave was:
then the wavelength did not change from T1 to T2, but the wave's position relative to the vertical line changed 1/4 wavelength, or 90 degrees. This change is called a "phase shift".