The 2nd part, "But some are useful" - simplifications of reality can be quite useful. They can help us explain, predict and understand the universe and all its various components. The idea of this quote is that every single model will be wrong, meaning that it will never represent the exact real behaviour.
When carbon is in the atmosphere it is usually in the form of molecules of carbon dioxide that is released by living organisms such as humans, plants, animals etc and it is a gas.
