The two measuring systems we use are measurement by volume and measurement by weight. Volume means how much space the ingredient takes up.
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.