The basic principle of classical conditioning is the idea that a subject can be taught an "unnatural" behavior through repetition. Typically, in classical conditioning, a subject is taught to act in a given way when exposed to a neutral stimulus that would not usually make the subject act in that way.
The classic example of this is the experiment done by Ivan Pavlov with his dogs. This is the experiment from which we get the term "Pavlovian response." In this experiment, the gods were taught to act in a given way (to salivate) when they were exposed to a stimulus (the bell) that would not usually have made them salivate.
The principle behind this is that subjects can be made (through reptetition) to respond reflexively to a stimulus that ought not to cause that particular reaction.