Plain English is Standard English used mainly for those learning English or wanting to improve on it.No slang or jargon is used or technical phrases or buzzwords used as this will only confuse further.Simple English is basic, plain and standard form, used for clarity, brevity and education.
It means there is no use of acronyms, abbreviations, jargon and slang words. Making sure your information is clear to understand and that it can not be misinterpreted in any way. There are organisations which will proof-read documents for you to make sure it's clear and is in plain English.
Plain means simple and clear, so I think here is simple english, as it's an adjective which has several meanings, it's better you check in a dictionary .