It's a common phrase used in restaurants or when someone is asking another person what they want to eat or drink.
Breakdown of the phrase:
* Que: This is the interrogative word for "what"
* vas: This is the conjugated form of the verb "ir" (to go), meaning "you are going"
* a: This is the preposition "to"
* pedir: This is the verb "to order"
Therefore, the complete phrase translates to "What are you going to order?".