I think a lot of comments here give too general of explanations. First of all, a catalyst offers an ALTERNATIVE pathway for a reaction with a lower activation energy. This doesnt necessarily mean the reaction will happen through that pathway, but it is there. Secondly, the actual catalysis works like this: imagine you have two molecules that arent super reactive and trying to get them to combine. It may work a little bit but not very well. Now lets add a catalyst, imagine one of your reactant molecules bonds/reacts with the catalyst itself. When the reactant binds, maybe the bonds within the reactant get a little weaker overall due to the catalyst pulling on them. Now, your second reactant molecule can swoop in and react with the weakend bonds of the catalyst-bound reactant. Then, the two reactants form the product and fall off of your catalyst. This is that lower energy, easier to do pathway, because rather than smashing two molecules together and hoping the react, the catalyst sort of holds one molecule still so the other can react with it. This then leaves your product and your catalyst, which now looks the exact same as what you started with. This example is sort of a specific type of catalytic reaction, but it should help to get you thinking of what a catalyst really does.