What is meant by the statement cleaner than cole?

It's this argument saying that even though natural gasses are also bad for the environment, coal is worse.

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