Emmanouel is the Greek form of the Hebrew Immanuel — meaning 'God with us.' It appears once in the NT (Matthew 1:23) as a fulfillment quotation from Isaiah 7:14. This name encapsulates the entire theology of the Incarnation: God becoming flesh and dwelling among His people.
Isaiah's original prophecy of Immanuel in 7:14 had an immediate historical context (a sign to King Ahaz) but a meaning that exceeded any near-term fulfillment. Matthew sees its ultimate realization in Jesus — the virgin-born Son who literally embodies 'God with us.' The Gospel of Matthew brackets itself with this theme: it opens with 'God with us' (1:23) and closes with 'I am with you always, to the very end of the age' (28:20). The incarnation is the turning point of all history because it answers the most primal human question: is God accessible, present, knowable? Emmanouel answers: not merely present, but present as one of us. John 1:14 ('the Word became flesh and made his dwelling among us') is the full explication of this name.