Enzymes are proteins, which are macromolecules that perform many varied functions within the human body. The role of enzymes is to speed up the rates of reactions occurring in the body or to initiate reactions that normally would not take place.
Enzymes need a minimal amount of energy called activation energy to start working. These special proteins have a specific place for reactants, also called substrates, to bind called an active site. The active site is specific to the enzyme, similar to how a key fits into a lock. With rare exceptions, only a specific enzyme can catalyze a certain reaction.
Enzymes are particular about the conditions in which they work. The wrong pH or high temperatures can warp enzymes to the point where they no longer function. This is called "denaturing" the enzyme.