The term carbon footprint is how much carbon goes into the air because of something done by people (not by nature). Doing something that burns fuel will make carbon dioxide gas in the smoke. Carbon dioxide has carbon in it. Just as walking on the sand leaves a footprint, burning fuel leaves carbon dioxide in the air, which is called a carbon footprint.
[change] Where it comes from
Oil, gas, and coal are called "fossil fuels" because they come from fossils of ancient plants and animals. Those plants and animals had carbon in them. So when we burn oil, gas, and coal, the carbon becomes carbon dioxide and goes into the air as smoke. This makes pollution. It also makes the greenhouse effect, which means that the carbon dioxide makes the earth warmer, like a greenhouse. A carbon footprint is the carbon dioxide that is put into the air because of a person, a group of people, an event or a product.
The words "carbon footprint" are often seen in the news. They are used with the word sustainability, which means that we can keep doing something without making something bad happen. That is, if there is too much of a carbon footprint, then there is no sustainability. People and businesses are often asked to think about how much carbon they are putting into the air, and how this changes the environment and maybe causes global warming. This is often said about traveling by airplane or by car, because burning fuel makes carbon dioxide gas.