The “-” is mandatory when the subject is a pronoun and the subject and the verb are inverted because of the interrogative form:

Que fait-il ? Que manges-tu ? Qui êtes-vous ?

“Vas” does not mean “going” alone, there is not necessarily a one-to-one match between each word of a sentence in English and its translation in French. “Vas” is the verbal form in “où vas-tu”, so it carries the same meaning as “are going” in English.