King's English or Queen's English is simply the accent of the standard English language as used in England. It is also commonly referred to as Received Pronunciation. You sometimes may see it specifically defined as the accent of standard English used in the south of England, but native speakers throughout the country use it.
The definition of the accent refers specifically to the south of England because it is generally thought that the language rose from residents of southern England before spreading to London and mixing with speech elements from other parts of the country.
It has often been revered in England as the accent of wealthy, powerful, and important people. It actually became associated with spoiled rich kids - people who were born into priviliege but didn't do anything to earn their status. King's/Queen's English started carrying a negative connotation with a lot of people and schools across the country even became more relaxed with formal education, allowing the use of other regional accents.