Read Deuteronomy 6:4-9 for the surrounding biblical/liturgical context.
Moses' farewell speech in Deuteronomy chapter 6 contains the central statement of Israelite monotheism. Verse 4, six Hebrew words, is the declaration; verses 5-9 are the obligations that flow from it: love God totally, teach these words to your children, speak them at home and on the road, when you lie down and when you rise, bind them on your arm and between your eyes, and write them on the doorposts of your house.
The orthography of Deut 6:4 in every Torah scroll has two enlarged letters: the ע (ayin) at the end of shma and the ד (dalet) at the end of echad. Together they spell עֵד (ed, "witness"). The reader of the Shema is a witness; saying it is testimony.
The full liturgical Shema combines three biblical passages:
The three paragraphs together take ~3-4 minutes to say. The opening verse alone takes 5 seconds. Learn the first verse first, then add V'ahavta, then expand.
The word echad in Adonai echad is usually translated "one". But it is also the root of unity / unified-ness, not the numerical one (which can be yachid), but the integrative one. Maimonides reads it as: God is not a being you count among beings, God is the only true unity, and everything else's apparent oneness is borrowed from God's.
This is why the word is held long. Each second the speaker extends the syllable chad, the rabbis read as another moment of consciously affirming that there is no other power, not luck, not fate, not the various deities of the surrounding world. Just one.