The sign
King Ahaz of Judah was terrified. Two kings were marching on Jerusalem. Isaiah told him to ask God for a sign. Ahaz refused, so God gave one anyway:
"Therefore the Lord himself shall give you a sign. Behold a virgin shall conceive, and bear a son, and his name shall be called Emmanuel."
Isaias (Isaiah) 7:14 · around 735 BC
A young woman having a baby is not a sign. It happens every day. A virgin conceiving is a sign. Only God could do it.
The name
Emmanuel means "God with us." Isaiah doesn't mean a messenger, or a man sent by God. He means God himself, with us.
What happened
Seven hundred years later, in Nazareth, an angel appeared to a young woman named Mary and told her she would bear a son. Her question was the obvious one:
"And Mary said to the Angel: How shall this be done, because I know not man? And the Angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee, shall be called the Son of God."
Luke 1:34 to 35
Matthew, writing for Jewish readers, pointed straight back to Isaiah:
"Now all this was done that the word might be fulfilled which the Lord spoke by the prophet, saying: Behold a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us."
Matthew 1:22 to 23
It was there from the start
Go back to the very first promise of a saviour, spoken right after the fall of Adam and Eve:
"I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel."
Genesis 3:15
Everywhere else in the Bible, the seed belongs to the father. Here, the one who defeats the serpent is the seed of the woman. A child with a mother, and no human father.
Why this is a big deal
The first promise in the Bible hints at it. Isaiah spells it out and calls it a sign from God himself. And the child's name tells you who he is: God with us.