This means that when we place garbage in a landfill, it will not magically disappear. This garbage will, however, degrade or decompose. To degrade or decompose means that garbage will breakdown into smaller parts. Some garbage is biodegradable. Biodegradable means that it will breakdown by bacterial means when it is exposed to moisture and air.
Some garbage is photodegradable.
Photodegradable means that it will decompose when exposed to the ultraviolet radiation of the sun. If an item decomposed into components which are helpful or harmless to the environment, decomposition is a good thing. However, many items decompose into components which are harmful to the environment. These components work their way into the air that we breath and into our water supply and can do harm to people and other living things. A partial answer to the garbage problem is recycling.