What is the law of mass action?
The law of mass action basically means that the rate at which a reaction takes place depends on the concentration of the reactants in the reaction.
Thus, if you have a general reaction of the form:
A + B --> C
then the rate of the reaction, k, = [A][B] where the brackets indicate molar concentrations.
What this means is that if you increase the concentration of one or more of the reactants you can increase the rate at which the reactants combine to form the desired product.