The frequency of the sound determines the pitch .The higher the frequency , the more high pitched it will sound. if the frequency is very low, it will sound very low pitched.
This is affected by motion of a sound source because if the sound source moves faster, this makes the frequency higher( since frequency is the number of waves/revolutions/whatever per second)
So , if for example , the sound source makes 4 vibrations or whatever in a second , it will have a frequency of 4Hz , but if a sound source makes 8 vibrations or whatever in a second , it will have a frequency of 8Hz . And since pitch is determined by frequency , the sound pitch with a frequency of 8Hz will have a higher pitch than sound source with a frequency of 4Hz.