Executor node (PlanState)
Also called: PlanState, execution node, plan node at runtime
When the executor runs a plan, each plan node becomes a live executor node (a PlanState) that holds the running state for that step, its position, its buffers, its children. Each one knows how to return the next tuple when asked.
What this means
When the executor runs the plan, each node in the blueprint becomes a live object holding that step's running state, its position, its buffers, its child nodes. Ask it for the next row and it knows exactly how to produce one.
Why it matters operationally
The whole executor is a tree of these nodes pulled from the top, one tuple at a time. The per-node numbers in EXPLAIN ANALYZE, actual rows, loops, timing, are measurements of individual executor nodes, so reading them is reading what each node actually did.