What are "referred" speeds and "referred TET"?
Referred speed is a way of compensating for the fact that air density changes with temperature.  For example, let's say you are designing a new turbojet engine. You fire up your engine one warm sunny day in July, and note that when the shaft speed is 5,000 rpm, it's making 10,000 pounds of thrust.  But then later, on a cold winter evening in December, you run the engine to 5,000 rpm, and now it's making 11,000 pounds of thrust!  What happened?  Did your engine suddenly get more efficient?  No.  It's just that colder air is more dense.  So for that same 5,000 rpm, you are pumping a lot more mass of air. Now, where referred speed comes in.  You are going to have to tell the pilot of the plane how to set thrust.  i.e. he wants to get 10,000 pounds of thrust, no more and no less.  You can't just tell him to set 5,000 rpm, because that will be a different amount of thrust depending on the ambient temperature.  So you tell him to set a specific "referred speed".  If you go to the same referred speed, you'll always get the same amount of thrust, no matter what the temperature.

You should also look up corrected speeds.  That is a very similar concept.