What does the straight line in a transverse wave represent

Straight line in a transverse wave represents the mean position of oscillating energy parameter at which the instantaneous amplitude is zero.

Explanation:

A transverse wave is one in which the direction of propagation is perpendicular to the direction of oscillating energy vector. The transverse wave having a horizontal straight line represents the line of zero amplitude of oscillation. When in graphical representation the curve intercepts this horizontal line then the amplitude of the oscillation and the phase of the wave becomes whole number multiple of 2π and as we know that the periodic oscillations are represented as a sinusoidal function which leads the amplitude to zero.