The prophecy is applied to Mary and to Jesus by Matthew under the inspiration of the Holy Spirit. A decent man, Joseph, considered divorcing his betrothed. It is the angelic messenger from God who changes Joseph's perspective. It is very clear to Joseph from the angel that Mary is not pregnant by a human male but by the Spirit of God. Essentially, this prophecy serves to point out that Jesus was the promised Messiah to be with us and to save mankind from its sins.
It was the Lord who laid aside His great glory and humbled Himself in order to become a helpless and vulnerable newborn baby. In order for fallen humanity to be brought back into full fellowship with God, He had to endure humiliation, rejection, suffering, and pain, and ultimately, to die by the cruelest method of execution imagined by the evil minds of wicked men.
It is imperative that we rid ourselves of our commonplace apathy towards God's Words and recognize the truth of Christ as Lord of heaven and earth. To become Emmanuel, God with us, He unveiled His majestic heavenly glory and wrapped it in a mantle of human flesh. By shedding our filthy rags, we can become joint heirs with Christ and become children of God.

Dear Heavenly Father, thank You for keeping Your promises, especially when it was so costly for You. The more I know You, the more familiar I become with You in my daily life, not just as someone who I read about. In reading the story of the birth of Christ, I pray that I might see You with fresh eyes. As my understanding of You grows, may I love You more deeply. One day, I will see You face-to-face and know as I am known because You live in me. Thank You. 
In Jesus's name I pray. Amen.