Relative motion changes observed frequency
When a source of sound moves toward you, the wavefronts bunch up in front of it, reducing the distance between successive crests. You observe a shorter wavelength and thus a higher frequency (higher pitch). When the source recedes, the wavefronts spread out, increasing the wavelength and lowering the observed frequency. The fractional change in frequency is (1 + v_source/c_sound)^-1 for a receding source moving at speed v_source, or approximately f_observed = f_source * (c_sound +/- v_observer) / (c_sound -/- v_source) for observer and source both moving. For non-relativistic speeds, the effect is a few percent at most (a car siren drops by roughly 10 percent as it passes). The effect is symmetric: from the moving source's perspective, a stationary observer appears to be approaching or receding, producing the same frequency shift.
Cosmology and practical technology
Astronomers use Doppler redshift to measure how fast distant galaxies recede: a redshift (lower frequency, longer wavelength) indicates recession velocity and, combined with Hubble's law, estimates distance. The cosmic microwave background shows a dipole Doppler shift revealing that Earth moves relative to the CMB at roughly 370 km/s. Doppler radar measures wind speed and precipitation by detecting frequency shifts of radio waves bouncing off moving droplets and particles. Doppler ultrasound in medicine measures blood flow velocity in arteries and veins by sending sound waves and analyzing the frequency shift of echoes from moving red blood cells, allowing assessment of vascular health without invasive surgery.