You first want to flatten one side on a jointer and then plane the other side on a thickness planer. A jointer can take the bow or cup out of a board and make one surface flat but it cannot make the opposite face parallel to the first face. That's where the thickness planer comes in. If you run this board only through a thickness planer you will just end up with a thinner board that still has a bow in it. I hope this answers your question.
"What we hope ever to do with ease, we must first learn to do with diligence".
- Samuel Johnson