Means CO2 equivalent and is used as a conversion factor to describe how high a CO2 footprint a given substance has on the climate. You use CO2e because CO2 is the substance you have typically used when talking about climate, increased greenhouse effect and temperature rises. When we talk about greenhouse gases, we also talk about nitrous oxide (N2O) and methane (CH4), which have a much greater effect than CO2: 1 kg CO2 = 1 kg CO2; 1 kg CH24 = 84 kg CO2e; 1 kg N2O = 298 kg CO2e.