Here in Genesis is the first occurrence in chapter 16. The angel is a messenger directly from God. Victor Hamilton explains in his commentary The Book of Genesis, that this angel is a "visible manifestation of Yahweh that's essentially indistinguishable from Yahweh Himself... more a representation of God than a representative of God."
In Genesis chapter 16, we see Abram giving in to the impatience of his wife instead of waiting on God's promise. We understand this from Genesis chapter 2. The Bible explains to us that when one man and one woman are united in marriage they become one flesh (Genesis 2:24).