'Projects' can be thought of as restricted workspaces. Sources and datasets are both linked to specific projects when they are created, and users must be part of a project (and have the correct user permissions) to view any of the data linked to it.
Your company may have multiple different projects associated with it, based on projects, teams, business units or data restrictions. Users can be given access to multiple different projects that are linked to their company.
To understand more about projects, see here.