During photosynthesis, plants capture carbon dioxide from the atmosphere, and water from the soil to carry out this process. What happens with carbon atoms is that they, with the sunlight action, will go to form glucose, whose chemical formula we can see it in the following:
What those 6 numbers tell us, is that for each 6 carbon dioxide atoms, plus each 6 water molecules, the process will be carried out, producing one glucose molecule, plus 6 oxygen molecules.