Could you please explain it, i know they increase reaction rates but how?
A catalyst lowers the activation energy (the minimum amount of energy needed for a reaction to take place). This means that the particles can react with less energy than they needed before the catalyst was added. (Think about it: if the government lowered the legal age to buy cigarettes, then more people could legally buy cigarettes. Similarly, if we lower the amount of energy needed for particles to react, then more particles can react). Also, a catalyst provides a surface on which the reaction can take place. This increases the number of collisions between the particles of the substances that are reacting.
A catalyst provides an alternative route for the reaction, (maybe more steps than previously), but each step having a lower activation energy than the original uncatalysed reaction. This means that although there will be the same number of collisions per second (if the reaction is performed at the same temperature as before), a greater fraction of those collisions will result in a reaction - so there will be more reactions per second. In the case of a heterogeneous catalyst - e.g. a solid surface the change is that the first step is a bond to the surface which waekens some of the bonds in the reactants - again making a greater fraction of reactions result in reaction.
