Watson's experiment goes as follows: He conditioned an infant, Albert, who was previously not afraid of rats to become afraid and cry when seeing them by banging a pipe behind the head of the baby to make a loud and startling noise to upset the child when in vicinity of the rat. The theoretical point being made was that we actually can condition humans, or control them to behave in a certain way by reinforcing the desired conditioned response to result in the conditioned stimulus.
It was a huge finding for behaviouralists, giving supposed evidence to the fact that environmental factors are law in shaping behaviour, and that with manipulation of the environment we can get the behaviour we desire. In the relatively short history of psychology -- as compared to other disciplines -- it played a large part in continuing the debate between different schools of psychological though.
This is also another example of stimulus generalization as both classical conditioning and operant conditioning are processes that lead to learning as little Albert experienced with 2 pairs of stimuli ( rat & loud noise) and the operant conditioning (his behavior and response overtime).