Two people can send messages to each other. The sender speaks into the plastic ring in front of one of the dishes and the receiver listens at the ring in front of the other. Try having a conversation, even though you are 25 metres apart.
The receiver can hear the message clearly even though there is lots of background noise. Sound waves are produced in the air when you speak. They are caused by vibrations of your vocal chords which open and close rapidly, releasing puffs of air. These pressure pulses, called sound waves, spread out from the speaker in all directions. If sound waves strike a smooth surface they will be reflected. The curve of the dish focuses these reflections so that they travel in a straight line to the opposite dish.