What is a substrate in the context of enzyme reaction?

Enzyme-substrate specificity means that a substrate can fit into an enzyme similar to a key fitting into a lock. The active site of the enzyme is what determines its specificity. An enzyme can hence catalyze a reaction with a specific substrate, such as amylase catalyzing starch molecules. During these reactions, the substrate is held in a precise optimum position to create and break bonds, catalyzing the molecule.