"What are they eating?"
Translation:Họ đang ăn gì?
Not exactly. Vietnamese doesn't have tense and English -ing and Vietnamese đang both mark the continuous, which is an aspect rather than a tense. In Vietnamese, a verb on its own can apply to any time but can be clarified with various markers. I believe that when we get further into the grammar we will find đang used for continuous actions in the past etc too.