Reading PostgreSQL EXPLAIN and EXPLAIN ANALYZE Output

Every PostgreSQL performance conversation eventually lands on a question that sounds trivial: what does this EXPLAIN mean? The output is almost readable. There are node names in English, numbers that look familiar, and enough structure that you can guess at the intent. But if you're guessing, you're going to miss the signal that actually matters — and the difference between a plan that returns in 0.3 ms and one that returns in 400 ms is often one line of EXPLAIN output that looks like boilerplat