A project is your future website. It may consist of several pages (a portfolio, magazine, or a blog), or of just one page (a landing page or a longform article).
The difference is in the settings: styles (e.g. fonts, colors) and domains are assigned to the entire project. Thus, if you want to create several interactive stories using one style (fonts and colors) publish them to the same domain, one project would be enough. If instead, you want to build several independent pages with different styles and publish them to different domains (e.g. landing pages for different customers), then you need to create several projects.