They speed up the reaction process.
Basically, an enzyme is an organic catalyst. It acts as a reaction pathway for less activation energy to be used.
As some substrates are too large to be broken down physically into its constituents, the enzymes help out by reacting with them to either break them down (such as the process of HYDROLYSIS), and also the creation of other chemicals (the action of carbonic anhydrase in blood is a good example).
Hope this helps......