The expression, “born again” is taken from a conversation Jesus had with a man called Nicodemus who was a leader in Israel;
In reply Jesus declared, “I tell you the truth, no one can
see the kingdom of God unless he is born again.”
(John 3:3 NIV)
Nicodemus thought Jesus was talking about normal birth and did not understand what Jesus meant do he asked Jesus;
“How can a man be born when he is old?” Nicodemus asked. “Surely he
cannot enter a second time into his mother’s womb to be born!”
(John 3:4 NIV)
To which Jesus gave him the real clue to what he really meant by saying to Nicodemus;
“I tell you the truth, no one can enter the kingdom of God
unless he is born of water and the Spirit.
(John 3:5 NIV)
So what did Jesus mean? He meant that a person needs to have new life inside them. This life is available through the Spirit of the Living God. To be born again is to be born of the Spirit of God. This is available to a person through asking God to forgive their sins and by putting faith in Jesus having died for those sins.
This is done through prayer.