An "eye-opener" is an English idiom that is often used when someone is surprised by something he or she has just seen, heard, or experienced. It is also often associated with an event so momentous that the person who experiences it has his opinions or views forever altered. The implication behind the idiom is that someone has been blind to a certain fact or theory but has had his or her eyes opened to the truth. People often alter "eye-opener" to form "eye-opening," which has a similar meaning but is used as an adjective instead of a noun.
In the English language, it is rare for a person speaking to describe everything using the literal definition of the words that are chosen. By contrast, certain phrases take on a meaning that is far from literal but instead informed by popular usage in the culture. These phrases are known as idioms, which allow for speech that is both colorful and colloquial. When people say that something is an "eye-opener" for them, it means that it has surprised them and changed his or her way of thinking.
As an example of how this phrase might be used in a sentence, imagine someone who has just seen a documentary about a particularly poor part of the world that he had never really known existed. He might say, "That documentary was a real eye-opener for me." The idea of the sentence is that the documentary has figuratively opened the person's eyes to something new.
This phrase has a particular power attached to it, since it often refers to something that is more than just surprising. As a matter of fact, something that merits the use of this idiomatic expression is usually powerful enough as an experience to forever alter the person who uses the phrase. For example, imagine that someone says, "I've been blind to how some people are treated in this country, but his speech was a real eye-opener to me." The suggestion here is that the speaker has not only been startled by new information but perhaps even motivated to do something about it.
Some people use this idiom in its adjectival form, which is "eye-opening." Consider the sentence, "The way that he behaved today was eye-opening and I'm not sure I'll ever be able to trust him again." Like all idioms, there is some exaggeration inherent in this phrase, since an experience can't literally open one's eyes. This exaggeration is what helps to give "eye-opener" its potency as an idiom.