Rather,[this] thing is very close to you; it is in your mouth and in your heart, so that you can fulfill it.
כִּי קָרוֹב אֵלֶיךָ הַדָּבָר מְאֹד בְּפִיךָ וּבִלְבָבְךָ לַעֲשׂתוֹDevarim 30:14
Says the Rabbeinu Bechayei that this verse encapsulates all of the mitzvot in the Torah. We have mitzvot of the mouth like blessings and prayers, and abstaining from sinful speech as well. We have mitzvot of the heart like improving our inner being, and nurturing our relationship with Hashem. Then we have mitzvot that are carried out through action like wearing tefillin, holding the 4 species, eating matzah, sitting in a sukkah, and more.
These concepts are further reflected in the mishnah in Pirkei Avot (Ch 1: 2):
Shimon the Righteous was among the last surviving members of the Great assembly. He would say: The world stands on three things: Torah, the service of G-d, and deeds of kindnessWhere we see that "your mouth" represents Torah, for we should strive to constantly keep Torah on our lips. "Your heart" represents one's efforts to influence one's heart, and draw closer to Hashem. "You can fulfill it" is realized by deeds of kindness, showing how mitzvot are brought about through various holy actions.
Ultimately this comes back to ideas we are exploring in the days leading up to Rosh Hashanah. The Rambam says that one of the essential activities of Tshuvah (Repentance) is viduy (confessing). Viduy is carried out by actually stating one's sins, elaborating and sincerely committing to not do them again. We see this in "your mouth" as the vehicle for expressing viduy, and "your heart" as the mechanism for feeling deep and honest sense of repentance.
As we lead up to Rosh Hashanah, we should all merit to experience complete tshuvah, grow in our relationship with Hashem, and be inscribed for a wonderful year. Shana Tova.