What is "Consume on/off?"

"Consume" on or off changes the behavior of how tracks either remain in or are removed from the Queue after being played.

If Consume is set to "on," then tracks will be REMOVED from the Queue after they are played. In that case, as an album or playlist commences, each track will disappear from the Queue after being played. If it is left to play from start to finish without any input from the listener, the Queue will be empty by the time the playlist or album has played to completion.

Leave Consume set to "off" if you wish for the previously played tracks to remain in the Queue, or if you have repeat mode engaged. 

You can turn Consume on or off by tapping the 3-dot icon at the top of the Queue window: