The conclusion is a freely editable field in the workshop settings (that is why you are redirected there). It is displayed on the Workshop main page once the activity is in the Closed phase. Teachers can use it to provide a common summary of the activity, what the students can take out of it etc. It serves the same purpose as the activity description, just displayed after the activity is over.
The red cross is just an indication that you have not filled that field. I agree that it actually looks as if you missed something essential, which is not the case of course.