The lock-and-key mechanism mentioned in this statement makes reference specifically to the way that enzymes, work on individual specific molecules to generate a specific reaction.
In essence it means that enzymes are like keys that only have specific locks in molecules and without the presence of this key, the locks, which would be the active binding sites where the key should go, remain inactive unless the enzyme is present at the site.
This lock-and-key mechanism was first described by Emil Fischer and it was proposed and widely accepted in the 1890´s as it explained how enzymes worked most clearly inside the digestive system to permit the absorption of nutrients by the body.
