Also called: sort order, path ordering
In plain English
Pathkeys are how the planner records the sort order a path produces its rows in. An index scan on (created_at) has pathkeys saying “already ordered by created_at”; a sequential scan has none.
Why it matters
Order is valuable. If a path already delivers rows in the order an ORDER BY, merge join, or GROUP BY needs, the planner can skip a sort entirely. That is why pathkeys are one of the dimensions on which a path can survive even when its total cost is higher — the free ordering pays for itself downstream.