This is the additional concentration of CO2 that would have approximately the same effect on
the radiative properties of the atmosphere - and thus the same direct effect on climate - as the concentrations shown of those GHGs.
The report then considers (a) how
the radiative properties of the atmosphere can be modified as a result, possibly leading to climate change, and (b) how the ozone layer could be modified, leading to changes in ultraviolet radiation reaching the Earth's surface.