In 1811, a long long time ago, Amedeo Avogadro performed many experiments with gases and found that equal volumes of gases are the same temperature, and pressure contain equal number of molecules.  As a  result, if the temperature and pressure of a gas is held constant, the volume of any given gas varies directly with the number of molecules.

To verify this idea, please click below:
Demo #6: Molar Volume

For example, if we take a close look at a chemical reaction that involves reacting hydrogen and chlorine to produce hydrogen chloride, we can use Avogadro's Law and state that equal volumesof hydrogen and chlorine contain the same number of molecules.  As result, we can assume that the chemical reaction can be interpreted as following:

    H₂ +     Cl₂         --->     2HCl

1 molecule    1 molecule     2 molecules
1mol            1mol             2mols
1 volume     1 volume        2 volumes

As a result, this simplification helps us solve gas problems that involve stoichiometry.

Avogadro's Law also indicates that gas volume is directly proportional to the amount of gas as long as the temperature and pressure are held constant.  Therefore, the greater the volume, the more moles of gas there will be in the gas.  This makes sense because if you have more stuff, you need more space to keep it contained in a certain container!