Plain English is a generic term for communication styles, which emphasise clarity, brevity and the avoidance of technical language. It is sometimes referred as plain language.
Plain language is understood the first time you read or hear it. It just means speaking and writing plainly so that all can understand!
Plain English is English written to be understood. It is wriiten in an appropripriate manner for the understanding of the audience. It is writing where there is no confusion about meaning, is free of cliche and unnecessary jargon. Various campaigns have launched the promotion of plain English in the UK and the USA since the 1970s, particulary focused on the official documents and publications and the language of law.
(Reference - Wikipedia/Plain English)