It’s a collective unit for all greenhouse gases that contribute to your carbon footprint. Fossil combustion primarily gives rise to carbon dioxide, but other greenhouse gases exist. Methane, for instance, is a potent greenhouse gas that cows produce a lot of.
It’s more short-lived than carbon dioxide but has a greenhouse effect that’s around 30 times higher. To include the impact of all greenhouse gases into one measure, they’re converted to carbon dioxide equivalents (CO2e).