The Doppler effect is the apparent change in frequency of an electromagnetic signal (radio or light) received by a moving observer relative to a static emitting source or by a static observer relative to a moving source. The apparent variation in frequency is proportional to the relative speed between the observer and the source along the path that separates them. The Doppler effect is used in astrophysics to find the angular speed of stars, because according to the perspective, their motion causes a shift in the radiation emitted compared with the same radiation emitted on Earth.
The sound from a moving object appears more or less high in pitch according to the speed at which it is approaching, and more or less low in pitch according to the speed at which it is moving away.
An analogy can be made with a stream into which leaves are thrown at regular intervals. On walking upstream (towards the source), you will see leaves transported by the current more and more often, and the more so if you accelerate. On the other hand, on walking downstream (away from the source) you will see leaves less and less often, until you see only one if you are moving at the same speed as the current. The high-pitched sounds are at high frequencies, i.e. you "meet" the wave often: you are approaching the source (or it is approaching you); the low-pitched sounds are at lower frequencies: you or the source are moving away. The principle is the same for light, which shifts towards longer wavelengths when moving away, and towards shorter wavelengths when approaching.
The speed of a moving body emitting a sound or light is determined by measuring the shift in the received frequency compared with the emitted frequency.