Return a value to the parent of this task i.e. the one that started it via Perform Task.
The parent task must have started it's child at equal or higher priority and have specified a Return Value Variable in it's Perform Task action, which will then be set to the Value specified in this Return action.
If Stop is checked, this task (the child) will terminate after returning the Value to the parent.
See Flow Control in the Userguide for more info.