This is a common phrase used to ask about the actions of a group of people. It can be translated literally as "What do they do?" but in English, it's more natural to use the present continuous tense ("What are they doing?").
Here are some examples:
* ¿Qué hacen tus amigos en el parque? (What are your friends doing at the park?)
* ¿Qué hacen los niños en la escuela? (What are the children doing at school?)
* ¿Qué hacen los trabajadores en la fábrica? (What are the workers doing at the factory?)