It's bucket sort, and parent is suggesting insertion sort within each bucket as the pre-merge step.

Say you're sorting alphabetically by title. You make one bucket per letter- put books into their respective bucket based on first letter of title. Then sort using insertion sort within each bucket. Then you're done.

(insertion sort is the human natural one- look for where the next one goes based on what you have so far and insert)

Same works for other heuristics.