A project, otherwise known as a document, is all the digital "stuff" within something you're writing.
A document has associated assets such as unlimited tabs, a note library and an image library.
For instance, you might make a document called "Blog Posts About SEO." You could then fill your note library with research on SEO and your image library with screenshots, graphs, etc about SEO. You could create innumerable tabs within this document, where each tab is its own blog post.
Or, as we do at Copy Hackers, you can use a document's tabs to create the various parts of a single campaign (e.g. tab #1: blog post & tab #2: cold email sharing the blog post tab #4: email to mailing list highlighting the blog post).
Still others use documents to create and organize their landing pages, academic papers or research reports.