[whpalmer4]

Invisible tasks are simply those not shown with your current view bar settings. The most common instance would be actions which you have completed. However, if you've got a more restrictive view bar setting (only showing available actions in a sequential project, or in a project with actions containing a future start date) invisible actions might be actions that have not been completed, and so OmniFocus warns you instead of just obediently deleting them. That gives you the chance to make sure you're only deleting what you want to delete.