Let's break down the sentence:
* Que: This is an interrogative word, meaning "what."
* Vas: This is the informal second person singular form of the verb "ir" (to go). In this context, it's used with "a hacer" to mean "are going to."
* a hacer: This is the infinitive form of the verb "hacer" (to do).
* mañana: This means "tomorrow."
So, the entire phrase asks "What are you going to do tomorrow?"