An overview is the view you get when you stand back to look at a whole thing rather than at its separate parts. It's a passenger's view from an airplanea view that makes it hard to recognize particular buildings but shows patterns in the roads among the neighborhoods.
This chapter offers an overview of the English sentence. Because you speak English, you already understand many things about the way English sentences work. Here you'll see the four parts of the sentence working together, and you'll have an opportunity to use each part and come to your own conclusions. In Chapters 2 through 7, you'll be given definitions and explanations of the four sentence parts; the details will become clear when you focus on each sentence part in those chapters. In this overview, though, just watch the patterns that the parts make as they fit together into whole sentences.