The article provides a detailed guide on understanding and interpreting the output of the EXPLAIN command in PostgreSQL, focusing on simple query plans. It uses a flight bookings database as a practical example to illustrate how EXPLAIN and its variations, like EXPLAIN ANALYZE and EXPLAIN (ANALYZE, BUFFERS), provide insights into the execution cost, row estimates, and data retrieval methods in SQL queries. The guide explains the significance of startup and execution costs, the role of nodes like "gather" and "parallel sequential scan" in the query plan tree, and how these influence query performance. By examining the query plan output, users can better understand how PostgreSQL executes queries, allowing for more efficient database querying and optimization.