As far as I can tell, the "Play queue" (upper left column) displays a list of the songs you have instructed Spotify to play such as upcoming songs in radio mode or upcoming tracks within the current playlist that is playing. Spotify has both active and passive queues so that queued songs (played songs from a search) are played before "non-queued" songs (those from the playlist/radio that were cued prior to the searched song(s) that were queued). Thus, actively queuing a song adds it to the end of the queue, but before the rest of the passively queued songs.

Confused? Me too, but don't worry about it ;-)