The passage states that the phrase "These are the words" (אלה הדברים) in Devarim serves to clarify that only the admonitions and reproofs in this Book were spoken by Moshe on his own initiative, whereas in the previous four Books Moshe did not speak even a single word from himself but rather transmitted the words of the Divine command exactly as given without any change; the passage further explains that Moshe chose to rebuke the current generation for their ancestors' insubordination of his own volition, which he was not commanded to do.
אֵלֶּה הַדְּבָרִים וְגוֹ׳. ״אֵלֶּה״ מִעֵט הַקּוֹדֵם, פֵּרוּשׁ, לְפִי שֶׁאָמַר ״אֲשֶׁר דִּבֶּר מֹשֶׁה״ שֶׁהֵם דִּבְרֵי עַצְמוֹ, שֶׁכָּל הַסֵּפֶר תּוֹכָחוֹת הֵם וּמוּסָר מִמֹּשֶׁה לָעוֹבֵר פִּי ה׳, וְאָמְרוּ זִכְרוֹנָם לִבְרָכָה (מגילה לא:) קְלָלוֹת שֶׁבְּמִשְׁנֵה תּוֹרָה מֹשֶׁה מִפִּי עַצְמוֹ אֲמָרָן, וַאֲפִלּוּ מַה שֶׁחָזַר וּפֵרֵשׁ מַאַמְרֵי ה׳ הַקּוֹדְמִין לֹא נִצְטַוָּה עֲשׂוֹת כֵּן אֶלָּא מֵעַצְמוֹ חָזַר הַדְּבָרִים, וְחָשׁ הַכָּתוּב לוֹמַר כִּי כְּדֶרֶךְ שֶׁאָמַר מֹשֶׁה מִפִּי עַצְמוֹ דְּבָרִים כָּאֵלֶּה, כְּמוֹ כֵן בַּמַּאֲמָרִים הַקּוֹדְמִין אָמַר מֹשֶׁה מִפִּי עַצְמוֹ אֵיזֶה דָּבָר, לָזֶה אָמַר אֵלֶּה הַדְּבָרִים פֵּרוּשׁ, אֵלֶּה לְבַד הֵם הַדְּבָרִים אֲשֶׁר דִּבֶּר מֹשֶׁה דִּבְרֵי עַצְמוֹ, אֲבָל כָּל הַקּוֹדֵם בְּאַרְבַּעַת חוּמָּשִׁים לֹא אָמַר אֲפִלּוּ אוֹת אַחַת מֵעַצְמוֹ, אֶלָּא הַדְּבָרִים שֶׁיָּצְאוּ מִפִּי הַמְּצַוֶּה כְּצוּרָתָן בְּלֹא שׁוּם שִׁנּוּי אֲפִלּוּ אוֹת אַחַת יְתֵרָה אוֹ חֲסֵרָה. אֲשֶׁר דִּבֶּר מֹשֶׁה. פֵּרוּשׁ, לְפִי שֶׁמָּצִינוּ שֶׁהוֹכִיחַ יִשְׂרָאֵל עַל מַה שֶׁהִמְרוּ עַל יָם בְּיַם סוּף, וְיֹאמַר הָאוֹמֵר: וַהֲלֹא אוֹתָם שֶׁהִמְרוּ עַל יַם סוּף כְּבָר מֵתוּ וְנִשְׁלְמוּ בְּט״ו בְּאָב שֶׁל שְׁנַת הָאַרְבָּעִים (איכה רבתי, פתיחתא) קוֹדֶם מַאֲמָר זֶה שֶׁל מֹשֶׁה, וְאִם כֵּן אֵלֶּה הַצֹּאן מֶה עָשׂוּ שֶׁמּוֹכִיחָם? לָזֶה אָמַר אֲשֶׁר דִּבֶּר מֹשֶׁה, פֵּרוּשׁ אִם הָיוּ הַדְּבָרִים מִפִּי ה׳ אֵין מָקוֹם לְדַבֵּר לָהֶם קָשׁוֹת, אֲבָל מֹשֶׁה הוּא הַמְּדַבֵּר וְיָכוֹל לְיַסֵּר גַּם אוֹתָם שֶׁהָיוּ אָז פָּחוֹת מִבֶּן עֶשְׂרִים שֶׁלֹּא נִגְזְרָה גְּזֵרָה, כִּי בֵּית דִּין שֶׁל מַטָּה מַעֲנִישִׁין מִבֶּן שְׁלֹשׁ עֶשְׂרֵה, וּכְנֶגֶד אוֹתָן שֶׁהָיוּ בֶּן שְׁלֹשׁ עֶשְׂרֵה בְּיַם סוּף הוּא שֶׁדִּבֵּר מֹשֶׁה קָשׁוֹת.
אלה הדברים, These are the words, etc. The word אלה is restrictive, especially in regard to what has been written previously. Seeing Moses recorded in this Book only words which he had spoken on his own initiative, the Torah wishes to emphasise that only the words of admonition recorded in this Book were spoken by Moses on his own initiative. We are told in Megillah 31 that Moses personally composed the curses recorded in this Book, and that even legislation which Moses repeated in this Book he had not been commanded to repeat but did so of his own volition. The Torah was concerned that we might conclude that just as Moses had felt free to say things of his own volition in this Book he might have done so in the previous four Books. This is why this Book commences with the words אלה הדברים, "only these are the words Moses spoke of his own volition, none other." אשר דבר משה, which Moses spoke, etc. Inasmuch as Moses commences with hinting at Israel's insubordination already at the sea of reeds, the present generation might well have countered that there was no point in dragging up sins committed by people who had long since died. After all, the last of the 600,000 men whose death had been decreed as a result of the sin of the spies had died on the 15th of Av previously (compare beginning of Midrash Eycha Rabbati). The Torah therefore says that if Moses had spoken in G'd's name such a complaint might have been justified. However, he personally, was not bound by such considerations and he considered it important to remind those who had been younger than twenty at the time of the Exodus to look back on a string of insubordinations the people had been guilty of already at that stage of the Exodus. While it was quite true that the Celestial Court sentences only people over and above the age of 20, the same is not true of courts administered by judges on earth who consider males over 13 years as fully liable for their actions. When Moses "dragged" up these ancient sins, he had in mind all those who had been at least 13 years old at the time.