Emmanuel

When God invites you to ask for a sign, you don't treat the gift lightly, but this is what Ahaz does. It is a clear indication that he has lost touch and connection with God. His relationship with God is distant, legalistic, and fearful. And while he has quoted the same passage that Jesus quoted when Satan tempted Him to ask for a sign, the circumstances are completely different. When Jesus quoted it, Satan was asking for a sign, whereas it is God himself who invites Ahaz to ask for a sign. 

And so God, through Isaiah, gives the sign and seal of his promise:
"The virgin will conceive and give birth to a child and will call him Emmanuel."

Now we read this promise at Advent and Christmas, seeing it as a prophecy pointing to the coming of Jesus, but that is its final fulfilment. It's initial fulfilment can be explained by the rest of this chapter and the one that follows.

What Isaiah is promised Ahaz is:
Very soon a child will be born and before the child reaches "the age of responsibility" the Northern Kingdom of Pekah and the Aram of Rezah (the hyenas) will be destroyed by the Assyrians (the big bear) who will plunder the land.

In chapter 8, Isaiah will take a young prophetess as his wife and they will have a child, "Maher-Shalal-Hash-Baz" which means "quick to the plunder, swift to the spoil" but they also invoke the name "Emmanuel" which means "God is with us".

The child will grow up eating honey and curds, living rough off the land instead of enjoying the fruit of a settled agriculture because the Assyrians have invaded and plundered the land. The Assyrians will destroy the hyenas.  

For many this initial fulfilment is a far-cry from the final fulfilment. The Assyrians were the coming super-power, a blunt instrument to execute God's judgement on the hyenas, but the clear point that Isaiah is making is that they are God's agents and are fulfilling God's agenda and that God will be with Israel.

We will see this prophecy initially fulfilled before Isaiah's child is between 4–5 years of age: Pekah and Rezin will be defeated and a little later when Ahaz is gone and Hezekiah is on the throne and the Assyrians arrogantly come to attack Jerusalem, God delivers the city and pushes the Assyrians out of the land. (This interpretation is strengthened by the fact that Isaiah had had to bring his first son "Shear Jashub" ("A Remnant Shall Return") as a sign to meet with Ahaz)

Emmanuel means "God is with us"—He is not far off—He is as near to humanity as the birth of a child. This is a great comfort.