It depends who you ask. The guys who wrote the notes in the NIV study Bible (which I used to read) think that it refers initially to Isaiah's child. They speculate that his first wife died in childbirth, and that "the virgin" is the same as "the prophetess" in 8:3, who is his second wife. Emmanuel would then find it's inital fulfillment in Maher-Shalal-Hash-Baz.

Others think it refers initially to Hezekiah, Ahaz' son, who would be free of the vices which gripped his father, and would turn the nation back to the Lord. In both these cases, they would be an example of the "already-but-not-yet" principal, i.e. something is fulfilled initially as a type, with the fuller fulfillment coming later, in this case, in the person of Jesus Christ.

Then others reject both of these interpretations, and say it applied directly to Christ. They'll cite, for example, that Hezekiah was 9 or something already by the time this prophecy was made, etc. etc.

I'm not sure exactly where I stand.

(Latin phrase goes here.)