This is the volume of 1 mole of any gas. Its value depends upon the temperature and pressure at which it is measured. The Molar Volume at s.t.p. (273 K, 1 atm.) is 22.4 dm3. The Molar Volume at room temperature and pressure (298 K, 1 atm.) is 24 dm3.
See also Avogadro's Law.