Doppler's effect is the apparent change in frequency in waves due to relative motion between observer and source of waves. A scientist called Doppler found it in 19th century.
Doppler shift in sound waves is observable easily when a train or zeep with a siren sounds and moves away. Or, even when there is a fast wind blowing.
apparent frequency = ν' = ν₀ [ ν + v_observer ] / [ v - v_source ]
where v₀ = original frequency,
v = velocity of sound in still air + velocity of wind, if blowing
v_observer = velocity of observer
v_source = velocity of the source of sound
Doppler effect is also observed in light waves.