The typical news or commentary program format involves one or more people sitting at a desk and talking. When one is speaking, the most common camera action is to focus on them so that their face is center-screen, essentially a classic from-the-chest-up "bust" in terms of statuary. So in that situation, what we SEE is a the head of a person talking. Hence, "talking heads" for anyone who has that kind of job.
The term is often purely descriptive but can also be dismissive of the person as merely a mindless script-reader. That secondary nuance must be determined by context. For instance, Johnny Carson on the Tonight Show was a talking head, but good. Walter Cronkite was a talking head, but good. Then there are those who are popular for other reasons but not noted for their brilliance. I'll avoid naming names because in the latter context, that decision is somewhat subjective.
