Tracks are loaded in the Playlist. In normal track progression, when a song is over, the next one played is the one below the just-played one. The Queue is one way to change the order in which the songs are played.
It is very easy to make a Queue; just select one or more tracks and move in the center of Amarok, to the Pop-Up Dropper (PUD), or right-click and scroll to Queue Track.
As you select or add songs to the Queue each of them takes a number. The number indicates the order in which songs will play.
If you want to make any changes in the Queue you can use the Queue Editor. Start the Queue Editor from the menu in → .