He lock-and-key mechanism refers to

The lock-and-key mechanism was first proposed by Emil Fischer which described as the enzymatic reactions whereby an enzyme with a single substrate binds temporarily to form a substrate complex. Just like a lock and key the enzyme as the lock and the substrate as the key is said to fit together

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.

i think this picture will you.