Ivan Pavlov was a Russian physiologist. During his studies, he observed that dogs would begin to salivate whenever an assistant entered the room, even if no food was present.
Pavlov realized that the dogs had learned to associate the presence of the assistant with the arrival of food. He then conducted experiments where he would ring a bell or sound a metronome immediately before presenting the dogs with food. Over time, the dogs learned to associate the sound with the food, and would begin to salivate in response to the sound alone, even without the presence of food.
This research on dogs became an iconic example of classical conditioning and the comic references this.
A good example of a pavlov joke is the one where pavlov goes to a bar after work, and just as he's about to order the phone rings in the bar and he shouts "oh shit I forgot to feed the dogs!"
Pavlov was a primitive psychologist who investigated conditioning by conditioning his dogs to salivate whenever they heard the sound of a bell (since they had been trained to associate the bell with being given food)
Pavlov is the dude that did behavioral research on dogs.
He found that after a while of ringing a bell right before feeding some dogs, they'd learn to associate the bell with getting fed, and would salivate when the bell was rung even when no food was present or given.