is one of the forms of the どれ
family of words used to enquire about unknown things. In this case it asks about a place. Just like どれ
represents ‘which’, どこ
represents ‘which place’, or simply ‘where’.
どこ should not be confused with どっち, a similar word which is also sometimes translated as ‘where’. どっち focuses more on the direction that one is/is going, where どこ focuses purely on ‘which place’.