What is compost?
The composting process involves organic matter (plants and animals that were once living) being broken down into simpler compounds.
The result of this process is a soil-like humus. New life is nourished by this nutrient-rich humus.
"Life is a circle of eating and being eaten, the chemicals of one body passing on to form another." Living at Nature's Pace by Gene Logsdon
The circle of life
In order for life to be built up, life must also be broken down.
All life stems from the energy of the sun. Plants use sunlight to make their own food called carbohydrates. A complex web of life is built up from the sun's rays. Animals eat the plants. The plant-eating animals are prey to predatory animals.
Just as life is continuously built-up into a complex structure, it is also being broken back down into simple molecules. For instance, an apple on the ground might be partially consumed by a rabbit (to build up its strength). The remainder is left on the ground to rot. The process of rotting is also called decomposition. Decomposers break down the apple into smaller molecules. Decomposers make nutrients available for plants to use. Without decomposition, plants would not be able to use the nutrients locked up in the apple's complex form.