Weight measurement is how much something weighs, in pounds and ounces. You can measure any ingredient using either of these methods, but it's easiest and most accurate to measure liquid ingredients by volume and dry ingredients by weight. Dry ingredients don't all weigh the same, like liquids do. Two cups of breadcrumbs might weigh only 8 ounces. Liquids are always measured accurately by volume.
Liquid ingredients are cooperative and can always be accurately measured by volume.
Because a liquid flows easily, it fills a measuring cup or spoon the same way every time. You don't get air pockets with a liquid, as you can with a dry ingredient. A cup of water today is a cup of water tomorrow. The volume and weight of most liquids relate. When it comes to liquid measure, there is a close correlation between weight and volume.