Transmission occurs when electromagnetic energy is allowed to pass completely through an object, such as when sunlight passes through a glass window or through water in a lake or stream.
In nature, very few objects are good transmitters. Have you ever held a flashlight against your hand in a dark room? Do you see the light shinning through? Now hold the flashlight against your arm. Your arm is not much thicker than your hand, yet it is thick enough that light is not transmitted through to the other side.
A leaf is a great transmitter. A lot of light is able to pass through it and get captured in the process of photosynthesis. Water is probably the best natural transmitter. Light is able to penetrate several hundred feet below the surface of the ocean in the right conditions.