What is a 'doppler effect'?

Miscellaneous Forums/General Discussion/What is a 'doppler effect'?

I have pondered this question before.

I bought FarCry a long time ago - Bloody great game - and I am currently replaying it.

In the sound settings there is a tick box for 'Doppler Effect'. However, they don't actually tell you what it does.

I know it affects the sound - but what exactly is it - what is the point of it?