“The Torah, this mitzvah which I command you is not beyond unnatural or beyond you. It is not in heaven that we say who will go up to heaven to bring it down for us to teach us so that we can do it. It is not across the seas so we might say who will cross the sea and take it and teach it to us so that we can do it. It is very near to you. In your mouths and hearts to do it“ (30.12).
The simple meaning is that this refers to the commandments in general But Nachmanides, Ramban, says this refers to teshuvah, repentance. That is the sub plot of these speeches and although we are familiar with the idea of teshuvah, there is no explicit command to repent in the Torah. And Joseph Albo in his Book of Principles, agrees. But the Talmud has already said (Eruvin 55a) this refers to fixing times for studying Torah. It is accessible to everyone.
It could mean something completely different that Eliezer Berkovits refers to in his book “Not in Heaven.” Once the law was revealed to Moses on Sinai, its interpretation and expansion is in our hands, human hands. It reminds us of the story in the Talmud (Bava Metziah 59b) where Rebbi Yehoshua, in the minority of one, brings miracles and a heavenly voice to support his point of view. The rabbis in the majority reply that they pay no attention to heavenly signs because “the law is not in heaven.”