Enzymes are three dimensional proteins that combine with reactants of a chemical reaction in order to lower the amount of energy needed for a chemical reaction to take place. During a chemical reaction, reactant molecules undergo a chemical change to form product molecules.
As an enzyme encounters a substrate, the enzyme is able to flexibly mold itself in order to fit the substrate. Enzyme fit is said to be induced because the flexibility of the enzyme depends on the presence of a substrate molecule.
Substrate is another word for the reactants of a reaction interacting with an enzyme.
Induced fit is like a clasping handshake. The Enzyme changes shape to embrace the substrate more snugly.