What is E.L. Thorndike's law of effect?

Thorndike essentially put a cat in a cage with a button. Every time the cat pressed the button a door to the cage would open up and it would get some food. At first it would press the button by accident but once it began to associate the button with the door opening and getting food so began to press the button on purpose. Soon every time the cat pressed the button it would get an electric shock so would then associate the button with a consequence. Th cat was always positively or negatively reinforced by the food or electric shock. Skinner did a similar experiment with a rat.

Thorndike's law of effect pretty much states "that any behavior that is followed by pleasant consequences is likely to be repeated, and any behavior followed by unpleasant consequences is likely to be stopped."