In the context of waves, such as sound waves or electromagnetic waves, the curve represents the variation of the wave's amplitude over time or distance. The energy of the wave does travel in a straight line in the direction of propagation, but the wave itself exhibits a curved shape due to the oscillation of the particles or fields that carry the energy.
When we visualize a wave, we typically plot the amplitude of the wave as a function of time (for a time-domain representation) or distance (for a spatial representation). The curve shows how the wave's amplitude changes over the given domain. The shape of the curve represents the specific characteristics of the wave, such as its frequency, wavelength, and waveform.
For example, in the case of a sine wave, the curve represents the smooth oscillation of the wave's amplitude over time or distance. Other types of waves may have different curve shapes depending on their properties. Nonetheless, regardless of the curve's shape, the energy of the wave still travels in a straight line in the direction of propagation.