Take for example 2H2
the prefix denotes the number of free atoms or molecules of substance needed to balance the equation - i.e. it is an amount. The subscript denotes the number of atoms in that particular molecule.
with the hydrogen + oxygen -> water example I wrote above, the 2H2
means there are 2 x Hydrogen molecules which have the configuration of H-H (the subscript denotes that hydrogen is in molecular form - which has 2 H atoms).
SO: in your example we have 2 Al molecules + 3 Sulphur molecules reacting to give 1
molecule. You see? The subscripts are denoting that the Al and S form discrete molecular units which all have 2 Al atoms and 3 S atoms in them. And - you need 2 Al atoms and 3 S atoms to make it.
Hope that is clear and that it helps -