Plan tree
Also called: PlannedStmt, execution plan, plan nodes
The planner's output is a plan tree: a tree of operations, sequential scan, index scan, hash join, sort, that, executed bottom-up, produces your result. It is the finished strategy the executor will follow.
What this means
The planner's finished blueprint: a tree of concrete operations (scan this table, use that index, hash-join here, sort there) that, run from the bottom up, produces your result. The executor's whole job is to follow it, step by step.
Why it matters operationally
The plan tree is exactly what EXPLAIN prints. Reading it tells you which access methods and join strategies PostgreSQL chose, and comparing the planner's estimates with reality (via EXPLAIN ANALYZE) is the single most useful query-tuning skill.