Ivan Pavlov was a Russian physiologist who performed an experiment involving a dog, food, and a bell.
Whenever food is presented to a dog, it would (naturally) salivate. In his experiment, Pavlov combined the ring of a bell whenever he presented food to the dog.
After a number of repeats of this procedure, he rang the bell on its own without presenting any food. The bell on its own now caused an increase in salivation in the dog, as if the dog is 'conditioned' to associate the neutral ring of the bell to yummy food delivery.
This experiment in 1890s became one of the first learning theories in cognitive sciences, termed classical conditioning or pavlovian conditioning.