What is the meaning of Loss of signal within the right subclavial artery in a segment of 2-3 cm consistent with a high grade of stenosis?

Answer 1

It means "your artery is narrower than it should be."

It actually means more than that (it describes where, and to some exent how they know), but unless you're a doctor it's not likely to make much difference to you (and if you are one, then you should already know what it means, or you're not a very good doctor).

This is a fairly significant issue, so you'll probably want to pay attention to what your doctor tells you it means to you specifically.