In a chemical symbol or formula, the coefficient put in front represents that

In chemical equations, coefficients represent the number of atoms or molecules involved in a reaction. In the rusting process, represented by the equation:

$4 \mathrm{Fe} + 3 {\mathrm{O}}_2 + \mathrm{x} {\mathrm{H}}_2\mathrm{O} \to 2 {\mathrm{Fe}}_2{\mathrm{O}}_3.\mathrm{x} {\mathrm{H}}_2\mathrm{O}$

4 iron atoms combine with 3 oxygen molecules to form 2 molecules of iron oxide. The coefficient "x" in front of  oxygen represents the variable amount of water molecules.