The Literal Commandment
"Hear, O Israel! The LORD is our God, the LORD is one! You shall love the LORD your God with all your heart and with all your soul and with all your might. These words, which I am commanding you today, shall be on your heart. You shall teach them diligently to your sons and shall talk of them when you sit in your house and when you walk by the way and when you lie down and when you rise up. (Deuteronomy 6:4-7)
The literal commandment is that we should speak the words of this passage when we lie down and when we rise up.
"Shema" is the Hebrew word that begins verse 4: "Shema, Israel!"
Shema means to listen and obey. Since the word shema begins this passage, the words of these verses are called "The Shema".