Also called: commit hint bits, HEAP_XMIN_COMMITTED
In plain English
To know whether a row is visible, PostgreSQL must know whether the transaction that created or deleted it committed. Checking the commit log every time is expensive, so the first reader that resolves the answer writes it onto the row as a tiny flag — a hint bit. Later readers trust the flag and skip the lookup.
Why it matters
Setting hint bits modifies the page, so the first SELECT after a write can dirty buffers and cause extra I/O — a common surprise when a read-only query unexpectedly writes. It is harmless and self-correcting: once the bits are set, future reads are cheap.