In biology, a stimulus is anything that provokes a reaction from an organism. We have a lot of instinctive behavior patterns that determine how we respond to various stimuli. For example, sweet food is a stimulus. Our instincts tell us to seek out and consume sweet food, because in a state of nature a sweet food is going to be something good and healthy like fresh fruit or honey.
However, our instincts do not know that humans have learned how to refine sugar and produce lots of it very cheaply. We have the ability to make artificial foods that are far sweeter than anything found in nature. Our instincts, working through the taste buds and the pleasure centers of our brain, tell us that we should eat as much as possible of these hyper-sweet foods. These artificial foods, the ones that fool are instincts, are superstimuli.
A superstimulus is anything that people are attracted to because it is an artificially enhanced version of something that their instincts tell them to seek out. A candy bar is a superstimulus. An airbrushed picture of a fashion model is a superstimulus.
Recreational drugs, like caffeine and heroin, are an even more extreme form of superstimulus. They work by directly stimulating the pleasure centers of the brain, which are normally stimulated by doing healthy or productive things.
In order to live a healthy life, you should train yourself to identify and avoid superstimuli. They are almost always harmful, and if you become habituated to them, you enjoy natural things less and less.
As technology advances, we are learning how to make more and more kinds of superstimuli. This robot baby seal is a perfect example, an artificial creation that mimics something natural. People are strangely attracted to it, because it is designed to push just the right buttons. Expect more and more of this kind of thing in the future. This one is mostly harmless because it replaces animals*, but in a few decades we will have superstimulus robots that replace people by imitating, and exceeding, all of the qualities we look for in our friends, like kindness or attention or beauty.
I expect this to cause troubles and societal changes at least as big as the widespread availability of junk food. Today, the healthy people are those who are smart enough to avoid junk food. In the future, the psychologically healthy will be those who are smart enough to avoid robot companions.
*Nobody knows exactly why people are attracted to 'cute' fuzzy animals in the first place. I personally think that it is due to some kind of flaw in our brains, and that our pets are themselves a superstimulus. I have noticed that keeping pets is often a substitute for taking care of children and grandchildren. They seem to satisfy the instinct for nurturing in the same way that a candy bar satisfies the instinct to eat sweet food.