Received Pronunciation (or RP, or BBC English) is the name given to the standard accent of English used by the British Broadcasting Corporation. It is used most often in the south of England, and by some people all over Britain.
Foreign students of British English learn this kind of English at school. The British pronunciation of words given in dictionaries is in Received Pronunciation. Received Pronunciation is used because there are many different accents used in Britain, and people think it is easier to learn to understand one accent than to learn to understand the many different accents.
Received Pronunciation is non-rhotic. This means that in words ending with an 'r', for example car, the final 'r' is not pronounced. Many words have long vowel sounds. For example, the 'a' in 'bath' rhymes is the same as in 'far', not the same as in 'cat'.