What is an enzyme?
An enzyme is a protein that accelerates a reaction. It does this by lowering the activation energy of a reaction. For example, lactase is an enzyme that converts lactose into glucose and galactose.
What is a substrate?
A substrate is what an enzyme acts on. It's a reactant in a chemical reaction. In the case of lactase, lactose is the substrate.
What is an active site?
An active site is where an enzyme and substrate meet. The big white thing in the image is the enzyme. The active site is inside a groove of the enzyme, and the black molecule is the substrate. There are two parts to an active site, the binding site, and the catalytic site.
A binding site orients the substrate to prepare it for catalysis. There are multiple theories for how this works. One theory, the "Lock and key hypothesis", assumes every substrate perfectly fits its enzyme. The "Induced fit hypothesis" says that an active site is flexible and changes depending on the shape of the protein so it can fit it snugly.
Once the substrate is fit snugly in the binding site, catalysis begins. They react with the substrate to lower the activation energy, or energy required to begin the reaction. There are some residues in the catalytic site that help an enzyme catalyze a reaction. Firstly, they make act as donors or acceptors of protons. They can also stabilize charge buildup on the transition state, which is an intermediate point in the reaction with the highest potential energy.