Suppose we add a catalyst, catalyst C, to our reactants. Now, before we look at the mechanism by which it increases the rate, let's make sure we understand the fact that catalysts are not used up in reaction. In other words, if you add some catalyst to our reactants, you will get that same catalyst back at the end of your reaction. Now, that catalyst might react somehow with one of the reactants, maybe covalently or non covalent. In other words, it might buy to it and help them for the products. But at the end it will separate and you will be able to get your feed back.