God tells Ahaz to ask for a sign from Him, to prove that he will confound the attack against Judah:
“Ask thee a sign of the LORD thy God; ask it either in the depth, or in the height above.” — Isaiah 7:11
But, Ahaz was fearful of the Lord and did not want to ask anything in case he was in danger of tempting the Lord.
Isaiah then answers him, rebuking him for not answering the Lord, and says the following prophecy:
“Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.” — Isaiah 7:14
The name Immanuel literally means “God with us” (“God is with us”).
And, as Christian’s should know, Matthew 1:23 is the fullfilling of that prophecy, when Joseph was told that Mary would give birth to the Saviour.
“Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost. Then Joseph her husband, being a just [man,] and not willing to make her a publick example, was minded to put her away privily. But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost. And she shall bring forth a son, and thou shalt call his name JESUS: for he shall save his people from their sins. Now all this was done, that it might be fulfilled which was spoken of the Lord 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. Then Joseph being raised from sleep did as the angel of the Lord had bidden him, and took unto him his wife: And knew her not till she had brought forth her firstborn son: and he called his name JESUS.” — Matthew 1:18-25
“Emmanuel” performed by Lucia Micarelli & Chris Botti