The Right Honourable

The Right Honourable (sometimes written as "The Rt Hon.", "The Rt Hon" or "The Rt. Hon.") is a prefix which shows honour. It is given to certain people, for example some MP's, in the United Kingdom, Canada, Australia, New Zealand, the Anglophone Caribbean and other places which are members of the Commonwealth. It is sometimes given in other places, as well. Examples of this are The Right Honourable David Cameron PC MP, former Prime Minister of the United Kingdom, The Right Honourable Stephen Harper PC MP, former Prime Minister of Canada, and The Right Honourable John Key MP, former Prime Minister of New Zealand. 
In the present-day United Kingdom, the term is given to members of the Privy Council of the United Kingdom. In effect, these are present or past members of Parliament who have served as cabinet ministers in a government.