Non-renewable energy is the kind the earth will not replace on any timescale we can use. Coal, oil, and natural gas are the usual examples: you burn them, they are gone, and a new field will not grow next year the way a forest can. Uranium for nuclear power is often put in the same group, because the fuel is mined and used up. Though actually, 'the earth will never make more' is a little too strong — oil and coal did form from old plants and animals — but that took millions of years, so for human life they are one-time stores. The pair word is renewable: sun, wind, moving water, and plants grown for fuel. Be careful with the label itself. People argue about whether something 'counts' as renewable, and burning wood or making some biofuels can still harm the air and the land.