What is the purpose of the introduction in an essay?

The function of the Introduction is to serve as a 'map' of the essay, outlining to your reader the main argument and points which you develop in your essay.

The introduction gives the topic, its boundaries and why it is interesting. The essay’s main sections (structure). Why they come in that order (structural principle). How the author plans to draw the necessary conclusions from the information available (methodology).The Thesis Statement (usually a single sentence). Its premise (the general claim about the information available). Its conclusion (the consequences of the first claim).