a catalyst is a substance that increases the speed of reaction by lowering the activation energy or providing an alternative pathway of a lower activation energy(more likely for enzymes in bio).
it also doesnt take part in the reaction but in chemistry, there are homologous and homogenous catalysts where some may take part in the rxn BUT they are regenerated and are not used up
chem: more particles will have the energy to overcome the activation energy, resulting in a higher frequency of effective collision, so the rate of rxn ↑. (usually catalyst is transition metal)
bio: enzyme-substrate complex, where it is essentially what happens in the chemistry part above but the conclusion is that it takes lesser time for nutrients to be broken down and therefore digested.