Hardscape is the non-living structure in a tank — driftwood, rocks and branches — used to create territory, hiding places and a natural layout; some woods also release tannins that tint the water.
Beyond looks, hardscape gives fish security and breaks up sightlines, which reduces aggression in territorial species. Bristlenose plecos actually need driftwood to rasp on.
Driftwood can stain water tea-coloured (tannins) at first — harmless and even beneficial for some blackwater species — and can lower pH slightly.