In chemistry, 6O2 is six molecules of O2, which equals 12 atoms of oxygen. The "O2" is a molecule of oxygen gas, while the "6" indicates that there are six such molecules.
Oxygen is most stable when it is in groups of two atoms, hence O2 is oxygen at its most stable. With regard to mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. 6O2 is commonly seen in biology, as it is found in many equations of life. One example is 6CO2 + 6H2O + energy = C6H12O6 + 6O2. This is the equation for photosynthesis.