You just added your first item, an HRULE graph item, to your graph representing your threshold line. HRULE items are based on a fixed value, therefore you did not choose any data source, but instead entered your threshold Value into the value field. Now all of your graphs based on this template will show a light grey line at the value 50.

You are now going to add your green, yellow, and red areas by making use of your newly created CDEF items.