What does the lock and key hypothesis state?
The key (substrate) fits perfectly in the lock (enzyme).
The lock and key hypothesis states that the substrate fits perfectly into the enzyme, like a lock and a key would.
This is in contrast with the induced fit hypothesis, which states that both the substrate and the enzyme will deform a little to take on a shape that allows the enzyme to bind the substrate.
Since, the substrate binds to a specific enzyme, this hypothesis is meant for the substrate specific enzymes.