Sound waves from a stationary source spread uniformly in all directions at constant frequency.
The Doppler effect is the change in perceived frequency of a wave due to relative motion between the source and the observer. You hear it whenever a siren, horn, or engine passes by β the pitch is higher as it approaches and lower as it recedes.
Sound travels at a fixed speed (c β 343 m/s in air). When the source moves toward you, each successive wavefront is emitted from a closer position, so the wavefronts pile up in front β shorter wavelength, higher frequency. When the source moves away, the wavefronts spread out β longer wavelength, lower frequency.