The virgin birth is the doctrinal term referring to Mary giving birth to Jesus by the power of the Holy Ghost and without having had any physical relations with any man. It is clearly taught in the New Testament records of the birth of Jesus.
Matthew 1:18 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.
Luke 1:34-35 Then said Mary unto the angel, How shall this be, seeing I know not a man? And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
This doctrine is important because it is the way that God became a man and in Jesus Christ became the God-man. According to the scriptures, in Jesus "the Word was made flesh, and dwelt among us" (John 1:14). The mystery of godliness was that "God was manifest in the flesh" (1 Timothy 3:16). This is why the One who was born as Jesus Christ was also called "Emmanuel, which being interpreted is, God with us" (Matthew 1:23). He is both fully God and fully man. He has two natures--the human and the divine--in one person.
By being born and living as a man, Jesus came as the "second man" (1 Corinthians 15:47) and the "last Adam" (1 Corinthians 15:45). As such, all that was lost in the sin and fall of the first Adam will be restored in the last Adam (Romans 5:14-18). He came in the "likeness of sinful flesh" so that He might condemn "sin in the flesh" (Romans 8:3). By being both God and man, He is able as God to provide an effective sacrifice for our sins (Hebrews 9:23-28) and as man to sympathize with our weaknesses and temptations (Hebrews 4:14-16).
The clearest prophecy of the virgin birth is in Isaiah 7:14, which states:
"Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel."
Here, a virgin is to bear a son and that son will be called Immanuel which means "God with us." If the child is God with us and the mother is a virgin, we have a clear prophecy of the virgin birth.
However, there are other prophetic statements that certainly point to the virgin birth. Immediately after the fall of Adam and Eve, the Lord declared unto the serpent, "And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel" (Genesis 3:15). The woman is said to have a seed and her seed is to bruise the head of the serpent or devil. The seed of the woman is certainly Jesus Christ. But how is the virgin birth implied here? It is in the concept of the woman having a seed. In the Bible, the seed refers to the man's part in bringing forth a child--though seed can also refer simply to offspring (as in Leviticus 12:2). But here in Genesis 3:15, the One to come and bruise the serpent's head is specifically called the woman's seed. Clearly, the virgin birth is in mind.
Another possible reference to the virgin birth is in Jeremiah 31:22, which states, "How long wilt thou go about, O thou backsliding daughter? for the LORD hath created a new thing in the earth, A woman shall compass a man." The idea of a woman compassing a man is said to be a new thing in the earth created by the Lord. This must refer to much more than a natural birth and would match up with Jesus Christ being the woman's seed.
Isaiah 9:6 points to the dual nature of the Messiah:
Isaiah 9:6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.
Notice that the child is born. That is the natural way of things and points to the humanity of Christ. However, the son is given. Something given already exists. Here, that something is the Son and is called "The mighty God." He must be both God and man and His birth must take a form where the everlasting God can be born as a human child. That, of course, is the virgin birth.
As with any doctrine, there is much more that could be said. I just hope that this gives you a good introduction to the subject.