The Laws of Prayer /The Blessings of the Shema (58)

1. When reciting the Shema one must have the intention that by this recital one is fulfilling the obligation to recite the Shema. (Shulchan Aruch 60)

2. When saying the first verse of the Shema one must think about the explanation of the words that he is saying, and if he did not do so, he has not fulfilled his obligation. The reason that it is only the explanation of the first verse which affects the fulfilment of the commandment is because it contains the main part of the acceptance of the yoke of the kingship of heaven, and the uniqueness of Hashem. (Shulchan Aruch and Mishne Berurah ibid)