Cells carry out thousands of chemical reactions, and most would be far too slow at body temperature. Enzymes solve this. An enzyme is a biological catalyst: a protein that speeds up a reaction without being used up, so the same enzyme molecule can work again and again. Enzymes are not used up, which is why cells need only small amounts of them.
Some enzymes build large molecules from small ones. This is synthesis. Sugars are joined to make carbohydrates such as starch, amino acids are joined to make proteins, and fatty acids are joined with glycerol to make lipids.
Other enzymes break large molecules into small ones. In digestion, amylase is a carbohydrase that breaks down starch into sugars, protease breaks down proteins into amino acids, and lipase breaks down lipids into fatty acids and glycerol. The small molecules can then be absorbed and used by the body. Without these catalysts, the building and breakdown of food molecules would be too slow to keep us alive.