Also called: PlannedStmt, execution plan, plan nodes
In plain English
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.
Why it matters
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.