Features > Projects > Special Permissions
When you create a project you can choose if it's going to be a public project or only certain users can access it. Public meaning that there aren't any special restrictions to it. All the user rights still apply.
The project rights you have selected apply to all of the tasks that are connected to that project, too.
So even if a user has rights to add and edit projects in general, and has access to all of the resources, but you don't give them rights to your project, they won't be able to change any of the tasks you have created for the said project. A padlock in front of the task title symbolizes that.
User rights and project rights work hand in hand. So if you give access to your project for a certain user but they don't have access to the resources attached to the project, they are still limited by their resource rights. Administrators still can view and control everything.