One of the richest Advent texts is this one:
All of this happened to fulfill the Lord’s message through his prophet:

“Look! The virgin will conceive a child!
She will give birth to a son,
and he will be called Immanuel
(meaning, God is with us)” (Matthew 1:22-23).
Advent is about “God with us.” It is about Incarnation. But “God with us” is more than simply a proposition about Incarnation, a proposition about God doing the really unthinkable — taking up humanity by becoming human — a proposition about christological natures. “God with us” is more than that.

Immanuel is promise — promise given from Adam and Eve and Abraham on, promise expected from the days of Israel’s yearning for redemption, and promise now fulfilled.
Immanuel is redemption — God with us is the good news, the gospel story that God has now taken up our case completely, God identifying with us in order to redeem us. “Immanuel” explains “Yeshua” (YHWH saves).
Immanuel is mission — if God is with us by sending the Son, the Son is with us as carry on the mission of making disciples (Matthew 28:16-20). It is not accident that Matthew both begins his Gospel on this note: “God with us in promise” becomes “Jesus with us in mission.”