The hypodermic model refers to a theory that what we see and hear in the media controls our behavior. This model was first suggested in 1930s behaviorism but was actually considered pretty obsolete for the longest time. Only recently has there been a modern revival of the theory.
In the 1930s there was a Nazi idea that the mass public could be controlled and manipulated through media and this theory was tested out a bit as well. Used in the wrong way the hypodermic model could have some very negative consequences for the common people.