Field permission can be enabled or disabled per entity on role. For each setting you can enable or disable Visible and Editable separately. If Editable is enabled Visible setting is not in use, user can then always both view and edit the field.
For each entity you can enable or disable all fields in entity at the same time on top level. If you expand the list for an entity you can enable/disable fields individually. Click the small arrow symbol in front of entity name to expand the list. Top level showing a yellow symbol means that permissions in the list are mixed, contains both enabled and disabled field permissions.
Note that when you create a new role, all field permissions are initially disabled.