Standard English is the variety of English that suits, and is the accepted norm, for certain situations where clear meaning is very important - and especially where the audience is not personally known.
In formal speaking and listening situations you will almost naturally tend to use Standard English without thinking, for example, when giving a speech in a formal situation to a group of people you do not know.
In Standard English, you will not use slang words and will even avoid some colloquialisms, for example, you might avoid saying 'cool' and may say 'fantastic' instead.
The keys to presenting an effective speech are preparation and practice - and a way to keep your nerves in check. World leaders don't just stand up and make up a speech on the spot - they carefully prepare and practice beforehand.
An effective speech needs to:
Martin Luther King's 'I have a dream' speech hit all three of these points, and was instrumental in changing the behaviours and attitudes of the time.
I have a dream that my four little children will one day live in a nation where they will not be judged by the colour of their skin but by the content of their character. I have a dream today!
Martin Luther King