What do the number in front of the reactants and products in a balanced chemical equation stand for?

They are called coefficients. They are used to balance an equation by multiplying all the atoms in a chemical formula (e.g. NaCl). That's the only way we can make a chemical equation balanced because we are not allowed to change the little numbers (subscript, indicates the number of atoms of that element present in the compound) within chemical formulae. In calculations with reacting masses, we use the coefficients to work out the molar ratio.