During my (brief) time in university, I took a course in psychology which seemed to consist mainly of researchers chopping out bits of animals' brains and watching to see what the animals could no longer do. Chop out this bit and the cat can't see anymore. Chop out that bit and the mouse can't stop eating. You get the idea.
In one of the more imaginative scenes of torture, a dog is put into a contraption composed of two metal cages and a tunnel joining them. If the dog is in the first cage, an electric current is passed through the metal and the dog leaps violently around, yelping in pain. Eventually, either by inspiration or accident, the dog leaps into the tunnel and passes to the second cage, which is not being electrified. Then the researchers electrify the second cage and the dog, no slouch, quickly leaps back into the first cage. Over time, the dog can get quite sophisticated at avoiding pain, as evidenced by its reactions to an array of variations on the "electric torture" theme. This process of learning has been aptly termed avoidance training.
However, a second dog is placed into a cage from which there is no escape tunnel. The dog is electrified repeatedly, until eventually it just cowers in the corner, no longer even trying to avoid the shock. This dog is then transferred into the other contraption with the two cages and the tunnel. No matter what the researchers do, they cannot convince the dog to jump up and escape through the tunnel. Hence the term learned helplessness.
I sometimes wonder if something like this happens to people when they are relentlessly abused and shit on from birth.