PriorityQueue in Email Filing problem

I am reading Danny Mittal’s Java code to solve the Email Filing problem in USACO 2022 February Silver Contest. I do not understand the following line:

PriorityQueue pq = new PriorityQueue<>(Comparator.comparingInt(j → folder[j]));

Specifically, what is the sorting criteria (j → folder[j])? Can anyone please provide more information about this part?

Thanks a lot.