The setting refers to 'where' the event/story/action is taking place. This would involve describing weather if outdoors, scenery, architecture, place, building etc. It is as though you were drawing up a setting of a play you would need to know all the details of what it is supposed to look like. Do not spare details, all the adjectives contribute toward the mood of the setting, e.g. if outdoors was it thundering and stormy or hot and sunny? The setting in a book or play or poem creates the atmosphere or the ambience which contributes to a greater understanding of WHERE IT IS TAKING PLACE.
The plot refers to the storyline, it is that simple, WHAT HAPPENS? If you are asked to describe the PLOT you would simply describe WHAT happens. You would list events in chronological (time) order of the story. It is telling the story in stages/levels or steps.