If Superman comes into contact with Green Kryptonite, he instantly becomes very weak. With enough exposure, he could die. Kryptonite has this effect because of the way it interacts with Superman's cells.
Much of Superman's power comes from the Earth's yellow sun. His cells are like living photovoltaic, or solar, cells -- they can store the energy from sunlight. Inside a photovoltaic cell, light comes into contact with a semiconductor, like silicon. The light's energy releases electrons from the silicon, and an electric field forces them to flow in one direction. In this way, a solar cell produces electricity.
You could also compare Superman to a plant that uses photosynthesis to make its own food. Through photosynthesis, plants use energy from the sun to turn water and carbon dioxide into oxygen and sugar.
It's unclear exactly which method best describes the way Superman's cells use sunlight, if either of them describe it at all. It's also unclear exactly why light from a yellow sun affects Superman in a way that light from a red sun does not. If Rao were a red dwarf, the explanation could be simple -- yellow stars are bigger, brighter and hotter than red dwarfs. However, Rao is a red giant, meaning it is bigger, brighter, and a little cooler than the Earth's sun. Although we do not know precisely why yellow light is so important, we do know that Superman needs it in order to have super powers.
There are a few possible explanations for how Green Kryptonite keeps Superman from getting power from the sun:
- The kryptonite radiation might displace the solar radiation responsible for Superman's powers.
- Kryptonite's ionizing radiation might displace electrons in Superman's cells, preventing the sort of electron movement found in solar cells.
- Kryptonite radiation may interrupt some other organic process within Superman's body.
As a result, Superman can no longer take advantage of the powers Earth's yellow sun gives him -- he loses his powers, and he may die.