This is a quote from a story by H.G. Wells called "The Country of the Blind." The actual quote is, "In the Country of the Blind the One-Eyed Man is King." I apologize for not being able to provide a page number, but the version online is not paginated. This is a wonderful adventure story, well worth reading, and I don't want to give away too much about it, since I hope you will read it, but I can discuss the context of the quotation and its more general application.
The protagonist of the story, Nunez, is climbing in the Andes and is lost to his climbing party, falling over a thousand feet, miraculously landing uninjured. He finds himself in a mysterious land which has been cut off from the rest of the world for generations. There have always been rumors about this place, like Brigadoon or Shangri-La, but very little solid information. Nunez has heard this quotation, though.
The people of this valley are all blind, from birth, and this appears to be a genetic trait. Thus, Nunez is the only sighted person around, and while the people of the valley have a great deal of enhancement of their other senses and seem fairly functional, the fact remains that being sighted gives Nunez, at least in theory, a great deal of power. Being able to see in a land full of blind people confers a great advantage, and that is really the point of the quotation. Even if one were only partially sighted, one would have a huge advantage over blind people. This advantage, unsurprisingly, plays an important part in the plot.
In a more general way, this is a meaningful quote, I think, making us consider using the power of our various advantages over others for good or for evil. Should we be kings and queens who lord it over those without, or should we use our advantages for the good of all? Will those who do not have our advantages take kindly to our interference? These are some of the questions the quote raises for me.
My father read me this story when I was very young, maybe ten or eleven, but I have revisited it since and have shared it with many students. I really urge you to read it. It's a great story!