They increase the speed of that reaction by decreasing the activation energy of that reaction. Now recall that the activation energy is basically the amount of energy that must be inputted into our reaction to overcome that energy barrier, to basically reach that transition state and transform the reactants to the product. So to see what we mean, let's take a look at the following diagram. So the y axis is the energy, the x axis is the reaction progress, it's the time. So we have the reactants and this is the energy of the reactants. We have the products, this is the energy of the products.