A full-on book proposal is a lot of work. It used to be a necessity for finding a publisher or agent. It would include information such as an outline for your book, a sample chapter, research on similar books, who do you think your audience is, why would they be interested in this book, your resume and credentials for writing it, what format you think it should take and why, even cover art suggestions, and how you plan on supporting and publicizing it.
I've heard of books being sold on a one-sentence pitch over coffee. But then again, the full-on book proposal is a really valuable exercise for your book, no matter what. Because...it will help you to work out the project, your motivation, who you're hoping to reach with it, and how committed you are to it. It's a lot of work but if you're not willing to invest your time in it to vet your idea, it's better to discover it early because making a book takes an incredible amount of time and commitment.