An enzyme causes chemical reactions that can break down different substances.
Make the food simpler to digest.
It really depends on the specific function of the particular enzyme. Mostly, they break down food particles for easier absorption in the digestive system. The enzyme amylase, for example, breaks down amylose (a sugar in carbohydrates). Lactase breaks down lactose, a milk sugar.
An enzyme basically breakdown the molecule it act on into macromolecule. For example starch is been broken down by enzyme amylase into sugar (glucose) and glucose into amino acid.