The lock-and-key model refers to the way in which a substrate binds to an enzyme's active site. Similar to how a key has to be the correct one for a lock, no reaction takes place if an incorrect substrate tries to bind.
The active site of an enzyme is a specific region that receives the substrate. It possesses a unique shape that complements that of the substrate, allowing for specificity to only one or two compounds. The substrate binds to the active site, and a reaction takes place that ultimately causes the release of the formed product. Enzymes catalyse this reaction by facilitating chemical bond changes in the substrate through altering the distribution of electrons. Once the product has been released, the enzyme regenerates, ready for another reaction cycle.
The lock-and-key analogy sees this process as very specified, in that only a particular key can fit into the keyhole of the lock. If the key is any smaller, larger or simply a different shape, then it does not fit the keyhole, and a reaction cannot take place. This lock-and-key analogy was first describes by Emile Fischer in 1894, and there since have been other theories to explain the mechanics of enzyme reactions.