ANSWER: Your question is a very important one. The scripture you are referring to is part of the conversation Jesus had with a Pharisee named Nicodemus. This Pharisee first approached Jesus at night (in order to conceal his meeting from the other religious leaders) to tell him they (the Pharisees) KNOW he came from God because no one else can do the same miracles he does (John 3:2)! Jesus interjects by stating that unless a person is BORN AGAIN he cannot be in the Kingdom of God (verse 3).
" . . . unless anyone is born again, he cannot see the kingdom of God." 4. Nicodemus said to Him, "How can a man who is old be born? Can he enter his mother's womb a second time and be born? (of the flesh)" 5. Jesus answered, ". . . unless anyone has been born of water (human birth) and of Spirit (resurrection from the dead to eternal life), he cannot enter the kingdom . . . (John 3:3 - 5, HBFV)
Note that in John 3 there are three births discussed - they who are born of the flesh (verse 4), water and the spirit (both in verse 5).
In the world of Christianity the expression "born again" is generally used to refer to someone who has "accepted Jesus" and been baptized. Some say they believe in the doctrine of 'once saved, always saved' by this. It should be abundantly clear from the entire discussion between Nicodemus and Jesus in John 3 that there is a SECOND birth. Note Nicodemus asks if one can enter a SECOND time into his mother's womb. Jesus, however, is talking about a DIFFERENT womb.
The womb Jesus was referring to was the EARTH. He was discussing this womb when later in John he said the hour was coming when those who are in their graves will hear his voice and be resurrected (John 5:28 - 29).
One could say that a Christian who was symbolically buried in a watery grave at baptism (see Romans 6:3 - 4, Colossians 2:12 - 13, 3:9 - 10 and 2Corinthians 5:17) then resurrected from it has been reborn or BEGOTTEN (or "begun" as the Greek seems to indicate) of water, receiving the earnest of God's spirit for his mind, giving him the mind of Christ. However, we are most interested in the new start which will bring immortality to us. Jesus was born of the flesh as the seed of David. However, you will notice that he was the Son of God with POWER when he was resurrected from the dead (see Romans 1:3 - 4).
We will not have been "saved" from death until we are born again with spirit bodies by a resurrection out of the womb of the earth, as John 3:6 states. Those composed the spirit can be invisible or visible (John 3:8). The power to make us spirit beings comes from above, from God. While there are many other scriptures showing the resurrection as the birth of a new body, I hope this brief answer aids your studies.