Also called: HOT update, HOT chain
In plain English
When you update a row and change no indexed column, and the new version fits on the same page, PostgreSQL writes the new version without adding any index entry. The old version simply points to the new one, forming a chain. That new version is a Heap-Only Tuple, because the indexes never learn about it directly.
Why it matters
HOT updates are the main reason a heavily-updated table can stay healthy. Because no index entry is created, there is no index bloat, and pruning can later collapse the chain and reclaim space without touching any index. You enable more HOT updates by leaving free space on pages (fillfactor) and by keeping indexes off frequently-changed columns.