6CO2 is the chemical formula for six molecules of carbon dioxide. The chemical formula of carbon dioxide is CO2. When writing a chemical equation for a reaction, a numeral in front of a chemical formula indicates the number of molecules of the corresponding compound involved in the reaction.
A possible chemical equation in which the symbol "6CO2" arises is the balanced equation for photosynthesis in plants, which is 6CO2 + 6H2O → C6H12O6 + 6O2. This equation means that six molecules of carbon dioxide react with six molecules of water to form one molecule of glucose and six molecules of oxygen.