Also called: checkpointer process, CKPT
In plain English
The checkpointer is the background process that performs checkpoints: it flushes all dirty shared buffers to disk and writes a checkpoint WAL tuple/" title="Tuple">record, giving crash recovery a bounded place to start. It also absorbs fsync requests handed off by backends and spreads its writes over time to avoid I/O spikes.
Why it matters
Checkpoint behaviour is one of the biggest I/O levers in PostgreSQL. Tuning checkpoint_timeout, max_wal_size, and checkpoint_completion_target changes how often and how gently the checkpointer writes — the difference between smooth and spiky disk load.