A fossil fuel phase-out means that we stop using fossil fuels like oil, coal, and natural gas, as sources of energy. These sources of energy were formed a long time ago from dead plants and animals, and they take hundreds of millions of years to form. We are finding more and more ways to use renewable energy sources, like wind and solar power, that don't take so long to make. When we use renewable energy sources, we don't put so much pollution into our air, and it helps make sure that the Earth is a safe and happy place for everyone!