Doppler effect, is nothing but the increase in the frequency of sound or light or any other waves as the source and observer move towards each other. Say for example, the sound of the ambulance increases as it comes nearer to us which is an example of doppler effect. For the derivation please refer the below link, which explains it clearly - https://www.topperlearning.com/doubts-solutions/derive-an-expression-for-the-apparent-frequency-when-the-source-of-sound-is-moving-towards-the-listener-please-explain-the-whole-thing-ihm8fv88