Doppler Effect describes the way frequnecy of a wave appears to change if there's a relative motion between the source of the wave and the receiver of the wave.

Look at the picture below:

Have you ever heard the siren from an ambulance in these two situations? The situation on the left in the picture is when the ambulance is stationary / not moving. In this case no matter which side you stand on, you here the same sound with same frequency. However in case 2 on the right, the ambulance is moving from left to right relative to you. In this situation a pulse of high pressure (the sound wave) moves away from the moving source of the sound wave and traveles outward in a sphere, centered on the spot where the source is ready to emit other pulses, it has moved.Thus the second sound-wave sphere emitted is centered farther and farther from the original source. So when the ambulance drives past right in front of you from left to right you suddenly here a higher pictch (higher frequency sound), and when it gets past you and getting away the siren gradually has a lowe pitch.