One’s primary kavana must be for the first verse, because in saying it we accept the yoke of heaven, as the Torah says: “These words… must be in your heart” (Devarim 6:6). Therefore, one must concentrate in her heart on the words she recites in the first verse. If she did not focus on the words she recited, she did not perform the mitzva (Berakhot 13b; SA 60:5, 63:4). Even one who concentrates on the full meaning of every word must try not to think about other things while reciting the verse. However, be-di’avad, it seems that as long as she also pondered the meaning of the verse, she fulfilled the mitzva (Peninei Halakha: Prayer, ch. 15 n. 2).
This is the meaning on which one should focus: “Shema Yisrael” – “Hear O Israel”; the mitzva to accept God’s yoke is intended for the Jewish people, the nation whose purpose is to express faith in God’s singularity in the world. “Hashem” – “the Lord”; this word is not pronounced as written. It is written as the Tetragrammaton, God’s four-letter proper name, but it is pronounced “Adonai.” While reciting it, one should focus on the meaning of the word as pronounced – that He is Master of all – as well as the word as written – that He is eternal. “Elokeinu” – “our God”; He is powerful, omnipotent, the master of all forces, Who rules over us. “Eĥad” – “one”; a person should prolong the recitation of this word long enough to have kavana that God is the sole ruler of the whole world, heaven and earth and the four directions of the world. This meaning is hinted at in the numerical values of the letters of the word: alef – one: God is One; “ĥet” – eight: God rules over the seven heavens and the earth; “dalet” – four: God rules over the four directions (SA 61:6; and see MB 18).
Be-di’avad, even if one does not focus on the exact meaning of each word and name but understands their meaning generally – that they are about the acceptance of the yoke of heaven – she fulfills this mitzva (Peninei Halakha: Prayer, ch. 15 n. 3). However, if her mind wanders and she does not concentrate on even the general meaning of the words of the first verse, she does not fulfill the mitzva and must repeat the first verse with kavana in order to do so (MB 63:14; Kaf Ha-ĥayim 17-18; see Peninei Halakha: Prayer 15:6).
It is customary to recite the first verse aloud in order to inspire kavana, it is customary to read the first verse out loud. It is also customary to cover the eyes with the right hand so as to avoid looking at anything else that might disrupt concentration (SA 61:4-5; MB 17).