Projects are a lightweight container to organize your related story maps and data. A project might mean slightly different things to different teams, and that's exactly why we've kept it light and flexible.
An example of a project could be Web App, or Team Digital. The scale and organization of your projects is totally up to you and will depend on things like how you split work up between different teams in your organization, as well as how you split story maps across your products.
Each project can contain any number of story maps. You will also need to manage your project team to allow access to the right people across your organization.