The list queue is about priority: You'll learn all the words from your first list before learning any words from your second list (before any words from your third list, etc.)
Use the arrows to change the priority of any list:
If your first list has no words that are new to you, you'll be introduced to words from the next list that does have unstudied words.
When you finish learning all the words from a pre-made list, the list will disappear from your list queue, but you will continue to review all of the words you learned.
Personal lists will not disappear, as you might add new words to them in the future (unlike our pre-made lists which do not change).
This is cool: If multiple lists contain the same word, you will only "learn" it from the first list you encounter it in.