Reducing a Sauce Concentrates Flavor
Simmering a sauce or stock uncovered to "reduce" it evaporates water while leaving behind everything that isn't water — sugars, proteins, salt, aromatic compounds — so the remaining liquid gets both thicker and more intensely flavored at the same time. It's why a sauce that tastes flat and watery often just needs more time on the heat rather than more seasoning; adding salt to a sauce you're about to reduce further risks oversalting it as the water disappears, so season toward the end instead.
Sources & methodology: These tips draw on established food-science references —
Harold McGee's food-science writing, America's Test Kitchen /
Cook's Illustrated testing, Serious Eats'
technique reporting, USDA food safety guidance,
and peer-reviewed food-safety research (including UC Davis
cutting-board studies) — not reworded recipe-blog consensus.