Permissions define what an internal user or external partner is allowed to do with a particular folder and its contents.
A description of the permissions can be found in 11.2.1: Which permissions can be granted?.
A listing of preset permissions for specific folders can be found in in 11.2.2: Permissions for folders that already exist and in 11.2.3: Permissions for new folders.
You can set permissions by sharing E-Mails, address books, calendar, tasks, folders or files.