Can somebody explain to me the book called "On Writing"?
I assume you are referring to On Writing: A Memoir of the Craft, by Stephen King. I think the key to understanding this book is in the subtitle. These are King's thoughts on his life as a writer, but really, these thoughts are woven in with memories of his life and ideas about lots of other things, too. If you were hoping for just a book of instructions, you are probably disappointed, but this book is much more. One of the qualities I admire about King is how he shares so much of himself with his reading audience. He talks about his family and his own struggles, not just about writing.
You might want to think about the book as a series of essays and read each section on its own. Otherwise, the book probably does seem a bit disconnected. Just enjoy each little piece for what it has to offer you. Because King does have some great stories and some great advice, too.