Spoken by God to Moses on Mount Sinai as recorded in Exodus 20, this majestic statement begins the passage known as The Ten Commandments.
Following the Hebrew numbering of the Commandments as well as many modern Protestant divisions of the Ten, this would be the First Commandment. According to the Catholic numbering, it is the first half of the First Commandment, the second half being the bit about not having any gods before God.
It is, obviously, not really a command. It's a statement of identity. Were they not spoken by the Supreme Authority in Heaven and Earth, the lines that follow would simply be the Ten Suggestions, or the Ten Good Ideas If A Bit Unrealistic. Because I am who I am, God is saying, you should pay attention. It also reaffirms something essential about the One who is speaking them: He is a God of deliverance. The Commandments are not given to Israel to make them slaves again, but as a charter of their freedom from slavery. Henceforth they would not be in bondage to sin as Egypt was, but will be a free and holy nation that walks in the ways of the Lord.