Sh'ma means "hear" or "listen". It's the first word of a key prayer proclaiming the "oneness" or unity of God and testifying to a belief in a monotheistic God. So when people talk about the Sh'ma, they are referring to this prayer. Sometimes they are only referring to the first sentence (shown below with and without the diacritic marks that indicate vowels) and sometimes to that sentence and a few more verses that follow (Deuteronomy 6:4-9) There are also slight variations in interpretation or translation. The Sh'ma is often chanted or sung. There are many different tunes.