Projects are listed projects by default, which means that members of the workspace can see that a project exists and choose to join it. For users on the Advanced plan or in an enterprise workspace, they can also choose to restrict projects to be unlisted. Unlisted projects will not be visible to anyone who isn't explicitly added to the project, and cannot be joined without an invitation.
You can make a project unlisted by using the toggle in a project's
Access permissions popover, which you can access from the project listings page or from within the project.