Enzymes are biological catalysts acting inside an organism. They are basically proteins. They are made up of amino acids.
Every reaction has got a threshold value of energy that is needed by it to take place. This threshold value decides whether the reaction will occur immediately or not. Some of the reactions have a real high threshold value.
Enzymes reduce this threshold value and thereby promote the reaction uptil equilibrium is attained.
For example, taking the process of digestion into account, it would takes a few days or months to digest a few grams of food, if it was not for enzymes acting in there.