The Sun is the most important source of energy on Earth. Other sources include geothermal energy, gravity... possibly nuclear energy and wave energy should be considered separate energy sources. But the greatest in magnitude and availability is the Sun -- solar energy.
The solar energy that makes it through our atmosphere heats and lights land, water, and air -- and us -- directly. But that effect is pretty much just during the day.
Plants are the greatest processors of and storage units for solar energy on this planet. They use solar energy and the substances they absorb through air and soil to build their bodies, reproduce, and store energy for future use. In the process, they convert carbon dioxide in the atmosphere into free oxygen and carbohydrates stored in their bodies -- their major form of energy storage. When they drop their leaves or shed other parts of themselves, and certainly when they die and decompose, they return to the soil the substance of their bodies they had gotten from soil and air.
Animals, including humans, breathe in the oxygen released by plants, and eat the plants, taking in plant carbohydrates which provide the animals with energy. Animals also eat other animals, taking in their stored energy (in the form of carbohydrates and fats). Animals use the substance (matter) of the plants and animals they consume to grow and renew their bodies.
We are what we eat, all of us.
In the process of metabolism, animals convert oxygen and carbohydrates into energy that they use to do what they need to do, and into carbon dioxide, which they release into the atmosphere when they exhale. Animals excrete what they cannot use of what they have consumed, and eventually die. Their bodies feed other animals and various microbes and other living creatures, sooner or later returning to the soil.
There the plants can take in the substances provided by the excretions and bodies of animals, other plants, and other living creatures, even as they use the carbon dioxide released from animals' lungs, and the cycle continues.
Unto Thee we return a small portion of the bounty Thou hast given us.
Our excretions -- our urine and feces -- were our first, primal libations and oblations. Unless we and other animals and plants give back to the earth what we have taken from it, the soil is depleted. This refers to our excretions and our bodies themselves, when we have no more use for them. It also refers to the parts of our food (other animals and plants) that we do not consume, indeed all materials that came from the earth. The longer we take without giving back, the less the soil will contain of the nutrients we need, and the more it will be diminished in both quality and quantity.
If we use up water sources faster than they can be replenished, we will have less and less water to use. If we pollute what is left...
Earth our home is a closed system (more or less, practically speaking) in dynamic equilibrium.