fillfactor
Also called: table fillfactor, storage parameter
fillfactor is a per-table (or per-index) setting that tells PostgreSQL how full to pack each page when inserting. At the default 100 it fills pages completely; at 80 it leaves 20% of every page empty on purpose.
What this means
How full to pack each page on insert. At the default 100, Postgres fills pages right up; drop it to 80 and it deliberately leaves a fifth of every page empty. Why leave space? So later HOT updates have room to place the new row version on the same page instead of spilling onto another.
Why it matters operationally
That reserved space is where future updates can place new row versions on the same page, which is exactly the condition for a HOT update. On a heavily-updated table, lowering fillfactor trades a little disk for far less index bloat and far more effective pruning. On append-only tables it gives no benefit and just wastes space.