Also called: table fillfactor, storage parameter
In plain English
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.
Why it matters
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.