This feature is only available for projects where the workflow module is used.
You review drawings in a workflow and depending on the review result you want to assign the corresponding status to the attached drawings, e.g. the status approved. So far you could change the status of drawings only in the drawing management application and therefore you had you close the workflow first.
With the traffic lights button you can now directly change the status of drawings in an open workflow and assign the approval. Just mark a drawing (1), click on the traffic light button (2), chose the proper status (3) and confirm with ok (4).