This phrase comes from Devarim (Deuteronomy) 30:14:
"כִּי קָרוֹב אֵלֶיךָ הַדָּבָר מְאֹד, בְּפִיךָ וּבִלְבָבְךָ לַעֲשֹׂתוֹ"
"For this commandment is very close to you, in your mouth and in your heart, that you may do it."
This verse emphasizes that fulfilling the mitzvot (commandments) of Hashem is not beyond a person's reach or capability. The Torah teaches that a Jew has the innate ability to connect with Hashem and follow His will. It is "close" to a person, meaning it is accessible in both speech (beficha – in your mouth) and thought (bilvavcha – in your heart), leading to action (la'asoto – to do it).
In the context of the Tanya, this verse is used to highlight that serving Hashem with one's inner faculties—thought, speech, and action—is within the reach of every Jew, regardless of their spiritual state. The Alter Rebbe emphasizes that this closeness means that no one is too far from achieving a relationship with Hashem.