Living things carry out thousands of chemical reactions in their cells. Many of these would be far too slow at body temperature, around 37 °C, to keep an organism alive. Enzymes solve this problem. Enzymes act as catalysts in biological systems, so they are often called biological catalysts.
Enzymes are made of proteins and are found in living cells. Like any catalyst, an enzyme lowers the activation energy of a reaction so that it goes faster, and the enzyme is not used up. Each enzyme usually works on one type of reaction.
Digestion is a good example. Amylase is an enzyme in saliva that speeds up the breakdown of starch into sugars. Enzymes are also needed for respiration, and yeast uses enzymes to make alcohol in fermentation. Too much heat can damage an enzyme so that it stops working.