What is an exposition?

By definition from Wikipedia, exposition is one of the four rhetoric modes of discourse, along with argumentation, description and narration.Well the purpose of exposition is to inform, explain, analyze, or define.
Well my image of exposition is said to be murky in my memory for if I did remember clearly, I had done expository writing during Secondary 1 for a short story called 'The Scarlet Ibis' and it is written for an In-Class Assignment.
At that time, it was somehow my first contact with expository writing and from my understanding, expository writing is to write out your own stand about a certain issue and suuport your own stand with support from evidences that we obtain from sources that are authorised in the field we are discussing. Expository writing provides one with a platform to express one's thoughts about certain matters and this evaluate one's understanding of the depth of the matter to find out what is behind the scene rather what is superficial and easy to identify just at the glance at it. Well this supposedly what I come to understand of exposition from what I learnt in my experience from writing the one and only expository essay in Secondary 1.