you need a diagram! two scales, one for shutter speeds, one for apertures. Explain how the camera determines how much light is needed to expose the film (has a device to measure the quantity of light) , line up those settings and show how all the other combinations are equal in the quantity of light using Donald's explanation. Once they have a grasp of that, add in filmspeed and how that moves the reference point back and forth along the scales, then explain high/low shutters speeds, big/little aperture effects and how you need to stay with matching 'pairs' of shutter speed and aperture. For the moment, I'd let them believe their camera's meter! If they come back again, then you can try to explain that!!
Reminds me of trying to explain the game of cricket or Aussie Rules football to a foreigner... Seems simple enough having been brought up on them, but really quite complicated to explain, where one thing implicates on another.. and another... etc!