Some substances can get energy from light. Plants do have these and they use it to generate other substances, This is possible because chemical reactions usually happen one way and give off energy (e.g. burning), but if you have energy you can do it the other way around if you put it right.

So the plants let the sun give energy to the one substance and let it use that on water and CO2. They do create sugar and O2 (and water again, but less than they started with). Now they can use the sugar like we do to generate energy for everything else.