Proteins (most enzymes are proteins) are sensitive to their environment. You have already seen how changing the substrate concentration and the temperature can affect the efficiency of an enzyme's ability to catalyze a chemical reaction. The concentration of hydrogen ions is another environmental condition that can change the rate of product formation in an enzyme catalyzed chemical reaction.
Hydrogen ions are positively charged particles that interact with the negatively charged parts of protein macromolecules. These interactions can disrupt the delicate balance of forces which hold the protein in its correct three dimensional shape. Anything which alters the shape of the enzyme can also affect the active site of the enzyme and also change the ability to interact with the substrate.
The pH scale is a measure of the hydrogen ion concentration. It ranges in value from 1 (high hydrogen ion concentration) which is acidic, to 14 (low hydrogen ion concentration) which is basic.