What does Hz mean on TV's?

Hz probably means Hertz. It's an unit that describes how many times each second something happens.

Hertz is frequency of some wave like activity e.g. sound wave, light wave, etc.

Long ago, the scientific units of measurement were descriptive; for example, "one horsepower" referred to the power that one horse could generate. Then "they" (the "International Electrotechnical Commission") decided to name all the units systems in honor of various scientists and discoverers.

One "Hertz", named for Heinrich Hertz, is one cycle per second.