The context of this verse is that when Ahaz was King of Judah, King Rezin of Aram and Pekah, son of Remaliah, King of Israel marched up to fight against Jerusalem. These men were trying unsuccessfully to persuade Ahaz to join a coalition against Assyria which had strong designs on lands to the west. (See the NIV Study Bible.) The Lord sent Isaiah to Ahaz to keep Ahaz from forming a counter-alliance with Assyria. Isaiah brought Ahaz a message from the Lord telling Ahaz not to worry and to stand firm in his faith. Furthermore, the Lord told Ahaz to ask for a sign so that he could be assured that the prophecy would come true. Ahaz refused to ask for a sign, but the Lord gave him a sign anyway. And the sign was this, "The virgin will be with child and will give birth to a son, and will call him Immanuel.... But before the boy knows enough to reject the wrong and choose the right, the land of the two kings you dread will be laid waste. (Isaiah 7:14-16)
Many scholars have pointed out that the word often translated as "virgin" in this verse can also be translated simply as "young woman". So apparently what the Lord was telling Ahaz was that a child would be born to a young woman and once this child grew old enough to tell right from wrong (in other words, when he was 13 and thus at the age where he would be held accountable to know God's Law) the two kingdoms which Ahaz feared would be laid waste.
So what does this prophecy have to do with the Virgin Birth of Jesus? It would seem that many of the prophecies of the Old Testament (from the perspective of the New) had both a near fulfilment and a more distant, future fulfilment. It is possible that the near fulfilment of Isaiah 7:14 had to do with Isaiah's own child. But Matthew saw something different in this verse. That is why we read in Matthew 1:22-23, "And this took place to fulfil what the Lord had said through the prophet: 'The virgin will be with child and will give birth to a son, and they will call him Immanuel'--which means, 'God with us.'" Presumably Isaiah's child was conceived through normal means. But Matthew is at pains to make the point that this was not the case with Jesus. Referring to Joseph, Matthew says, "But he had no union with her until she gave birth to a son." (Matthew 1:25)
Of course, Luke also makes it quite clear that Mary was a virgin. He calls her that twice in Luke 1:27. And Mary refers to herself as a virgin when she asks the angel in Luke 1:34, "How will this be since I am a virgin?"
Tomorrow we will look at why this part of The Apostles' Creed is important.