In a chemical equation, the coefficients indicate the number of molecules while the subscripts describe the number of atoms in one molecule. See the following example: C6H12O6+ 6O2---> 6CO2+ 6H2O
The numbers 6, 12, and 6 in the first reactant are subscripts and indicate the number of carbon, hydrogen and oxygen atoms, respectively, in one molecule of glucose. The number 6's in front of O2, CO2, and H2O are coefficients and indicate that for one molecule of glucose, 6 molecules each of these substances are involved in the reaction. Sorry the text box doesn't allow me to type the subscripts. I hope this is not too confusing.