The idea of a "carbon footprint" became popular in the mid-2000's. A person's "carbon footprint" is the amount of carbon dioxide that is released into the Earth's atmosphere because of that person. Having a large carbon footprint is bad because it speeds up "climate change", which is also known as "global warming".
Some choices, like driving a large car that uses a lot of gasoline, increase your carbon footprint. Other choices "reduce" it:
One way to reduce your carbon footprint is to eliminate meat from your diet.