What is being born again?
The Bible gives the image of a field in which a seed falls.
The field is the human heart. The seed the word of God.
Someone hears the word of God, it falls in him on good soil and bears fruit.
1 Peter 1:23
You have been born again not of seed which is perishable but imperishable, that is, through the living and enduring word of God.
Again, faith, repentance, and conversion are (at least largely) the work of man, to become born again, is the work of God.
It is symbolized by water baptism