It's the volume occupied by 1 mole of gas at a specific temperature and pressure (STP).
The molar volume of a gas the volume occupied by 1 mole of the gas. However, because gas volumes change according to pressure and temperature, the molar volume is specified as the volume occupied by 1 mole at STP, which is 273K and 1 atm pressure.
It is nonsense to say thet "gases have no volume". Gases are composed of atoms or molecules, and both of these particles occupy space. Volume is the amount of space occupied by all of these. The only things that have "no volume" are things that are 1 or 2 dimensional!