Meaning that if any dispute arises in any matter, one should refer it to Allah, i.e. to the Book of Allah and the Prophet's  Sunnah.

The order is being given here in clear and explicit terms that whenever the people dispute in any matter, whether it concerns the principle of religion or any branch of religion, the only way of deciding it is to take Allah's Book and the Prophet's  Sunnah as the arbitrator. And whatever is met with therein should be accepted without question. So, whatever order is given by the Book and the Sunnah, and whichever problem is evidenced by them as true, that alone is right and true and all else is false.
Allah Most High says in the holy Quran: -

The instruction here too is the same that if you are true in believing in Allah and the Last Day, then of whichever matter you are not aware, or you have dissent in any proposition, or entertain divergent views in a matter, you should refer it to Allah's Book and the Prophet's Hadith for decision, and believe in whatever there is in both of them.

So, it is proved from this noble verse that the man who does not refer controversial matters for decision to the Book and the Sunnah does not believe in Allah and the Last Day. Similarly, the same has been shown to be the purpose of the revelation of the Quran in itself: