Also called: walwriter, WAL writer process
In plain English
The WAL writer flushes the write-ahead log to disk in the background on a regular cadence, so that busy backends and asynchronous commits don’t each have to fsync the WAL themselves. A backend committing synchronously still flushes its own WAL, but the WAL writer handles the steady background flushing.
Why it matters
It is most relevant with synchronous_commit = off, where commits return before their WAL is durable and the WAL writer (paced by wal_writer_delay) catches up shortly after — trading a small durability window for higher throughput.