For the older people like myself that don't understand why everything needs to be a 5-minute video, here's a 15-second read:
Classical conditioning (also Pavlovian conditioning) is a type of learning that happens subconsciously.
Ivan Pavlov (1849-1936) was the first to show the way in which it works. He did this in an experiment using dogs. Pavlov noticed that the dogs naturally salivated when they saw food. He paired this unconditioned stimulus (showing food to the dogs) with another, neutral stimulus: the ringing of a bell. Pavlov discovered that, if the two stimuli are presented together again and again, the organism learns that they belong together.