"Lean Publishing" is the act of publishing a book while you are writing it, chapter by chapter.
This allows you to get feedback from early readers, and can be a great way of motivating you to finish a long project, because you don't have to wait until the end for a response from your audience.
If you're interested in learning more about this process, and how it relates to conventional serial publishing, please see the following talk by Leanpub co-founder Peter Armstrong: