The Shema is central to the Jewish belief. It proclaims that Hashem is our G-d and that he is the one and only. Kaballah and Chassidus have much to say about this prayer.
The Alter Rebbe, founder of the Chabad Chassidic branch of Judaism wrote a great deal about Shema and the power it has to connect a Jew to their source, which is Hashem.
The following are excerpts from Ha Yom Yom as well as Ch. of Tanya. Meditating on these concepts before davening and during davening.
Tanya Ch. 41
Tefillin is connected to Shema as the Shema is part of the 4 sections of scripture contained inside.
“…That is to say, that he should use the wisdom and understanding that are in his soul, for G-d alone. Similarly, [he should intend that] the attribute of Daat in his soul, which includes both the Chesed and Gevurah, i.e., fear and love, in his heart, be nullified and absorbed into the attribute of the Higher Knowledge (Daat HaElyon), which comprises kindness and severity, and which is clothed in the passage of Shema and Vehayah im shamoa.
Ha Yom Yom 12 MarCheshvan
One of the Alter Rebbe’s early teachings, then called verter (See Sivan 15.) (lit. “words,” short sayings):
Sh’ma Yisrael: (Lit. “Hear, O Israel.”) a Jew (From the word Yisrael, one of Israel.) senses (From sh’ma, “hear,” “listen,” or “comprehend”; hence, “senses.”) that
Havayeh (See Tishrei 3 and footnotes.) Elokeinu – our strength and life (Or “vitality,” “life-force.”) is beyond nature, (Havayeh, G-d transcendent, above nature, is Elokeinu, “our G-d” – our strength and life. Viz. Tishrei 3.) and
Havayeh Echad – Havayeh is One.
Ha Yom Yom 19 MarCheshvan
A response of the Alter Rebbe in a yechidus: Chassidus is Sh’ma Yisrael. (Hear O Israel.) The word sh’ma is an acronym of s’u marom eineichem, “Raise your eyes on high. (Yeshayahu 40:26) The verse says “on high” (marom), and not “to the heavens” (shamayim). “On high” means higher and still higher, to attain a level beyond intellect, and grasp this itself intellectually – as the verse concludes, “and see Who created these.”