Autotrophs are organisms that are able to harvest the carbon they need directly from CO2. Autotrophs are the foundation of all life on earth, in that other organisms (the heterotrophs) are able to utilize only organic forms of carbon: carbon that has been fixed by autotrophs.
There are two classes of autotrophs: lithoautotrophs and photoautotrophs. Lithoautotrophs are able to "fix" CO2 using energy they obtain from respiration.
By far the larger and most important class (more important, that is, to the global environment), photoautotrophs are photosynthetic, which means that they use energy from the sun to break up CO2 molecules and make glucose, which they are then able to use to build up the organic matter that constitutes their own cellular matter and — ultimately — food for all other living organisms.