"What do you understand?"
Translation:Vous comprenez quoi ?
This was answered above by relox84 - It can be "Que comprends-tu ?" or "Qu'est-ce que tu comprends ?", the first one forms the question using the inversion "comprends-tu", and the second one forms the question by adding "est-ce que". See the Tips and Notes for Questions for all the different ways that you can form a question in French: https://www.duolingo.com/skill/fr/Questions/tips-and-notes