Explain what is meant by the statement cleaner than coal

(Clean coal is a concept for processes or approaches that mitigate emissions of carbon dioxide (CO2) and other greenhouse gases, and radioactive materials, that arise from the use of coal, mainly for electrical power generation, using clean coal technology.)

Environmental advocates don’t love natural gas, but some will at least reluctantly acknowledge that it’s cleaner than coal.

pretty much it's this argument that natural gas even though it's still bad Atleast it's not as bad as coal.