In a stereo mix all sounds have to be placed at some 'location' in the sound spectrum to create a stereo effect. In the early days of stereo mixing sometimes the instruments were placed at one end of the spectrum, and the vocals at the other end, a famous example is Rubber Soul by the Beatles. Since the late 60s the drums and lead vocals are placed in the middle most of the times. This placing of sounds is called 'panning'.
It is beautifully illustrated by some sketches Paul McCartney made during the mixing of his album Press To Play (1985). Each sketch is devoted to a song of the album by representing its sound cartography in the space of the recording studio.