'Standard English' is the variety of English that is considered by many to be the accepted form of English: it shows no variations (e.g. regional variations) that are considered by some to be ungrammatical or non-standard.
Standard English should be spoken with Received Pronunciation (RP): without regional variations. It emerged from the 18th and 19th century aristocracy and it continues to be the standard for spoken English. Standard English and RP are extensively used in the media, by politicians, by public servants, in government departments and educational institutions.
Next time you watch a news broadcast or a politician speaking on television, listen to their voices and see if you can spot any non-standard English - you should find there is very little, if any.
This should be an easy 11-plus quiz for you.