The Runbook Description field is presented both when Creating a Runbook and from the Runbook details screen. The description field has a limit of of 250 characters. When entering text into this field the character count in the bottom left of the text box will count the number of characters which have been added into the field. Once you have reached the limit of 250 characters the text within the box will turn red, highlighting you to the fact that you cannot enter any more information into the field.