As you probably know, plants produce glucose through the process of photosynthesis, which has the following equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

If this equation is not balanced it can seem confusing where the hydrogen comes from, as you would start with 2H in H₂O and end up with 12H in C₆H₁₂O₆. However, when the equation is balanced we can see that it takes 6 molecules of H₂O to make one molecule of C₆H₁₂O₆, which provides the necessary 12H. These hydrogen atoms are released from H₂O in an oxidation-reduction reaction driven by light, which is why plants need the sun to survive. This also explains why you get an extra 6O₂ on the right side of the reaction; these are the remaining O₂ molecules produced via oxidation. Hope this helps!