Translation:The car that's moving over there by the building is the fifth.
Here is a better explanation: unlike English, where relative clauses must follow the word the modify, this is not always possible to do in Hungarian. For example, sometimes you want to emphasize the "antecedent" (referred to word) of the relative clause. So in that case you can't put the relative clause after the antecedent, because the verb comes after the antecedent. Hungarian gets around this problem by requiring a target pronoun like az to mark the antecedent. Hence the car, which... in english becomes az az auto, ..., amelyik .... This target pronoun can be dropped if amelyik follows the antecedent directly, but only in that case.
This "az az autó", without the rest of the sentence, is simply the equivalent of "that car".
This is a very simple rule: this/that/these/those plus object will be translated as:
ez/az/ezek/azok + definite article + object
This car - "ez az autó"
That car - "az az autó"
These cars - "ezek az autók"
Those cars - "azok az autók"
This chair - "ez a szék"
That chair - "az a szék"
These chairs - "ezek a székek"
Those chairs - "azok a székek"
So: "That car is the fifth, which ...." - "which" referring back to "that".
Except we don't need to do this in English, it would sound weird.