According to Jamaal `Ataaya, in his book حقيقة النسخ وطلاقة النص في القرآن, pages 370-371, Ash-Shawkaani wrote in his book فتح القدير, volume 2, page 422, that the meaning of
is that the verses of the Quran are fixed, perfected, without fault, undemolishable, like an established building. Some said it means they have not been abrogated like the Torah and the Gospel were.
My humble comment, which I expressed before, is that it's not the Torah and the Gospel that were abrogated, but the Old and New Testaments; and it wasn't abrogation, but naskh. The difference being that abrogation is annulment while naskh is correction and restoration of the original.
`Ataaya adds a gem: he says that the addressees in the completion verse,
are all mankind! This verse is a declaration from God to humanity that His religion is Islam and the scripture is complete and fulfilled. One of the reasons the Quran was revealed is to correct all the false doctrines that people developed over the centuries and by which they changed the verses and laws of God. The Quran therefore abrogates everything that differs from it and contains nothing that was or can be abrogated. I like that to be the subtitle of a book I like to write about the abrogation issue:
القرآن ناسخ لما خالفه ولا منسوخ فيه