People at my church say born again constantly and I just nod along. What is that actually supposed to mean?

“A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.”

Ezekiel was a priest deported to Babylon, speaking to exiles who knew their own history of failure far too well to feel optimistic about themselves. What he brings them is not a better set of rules. It is replacement surgery: a new heart, performed by God, on people with no track record to recommend them. Being made new was God's idea long before it became Christian vocabulary.

“Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.”

Nicodemus came to Jesus at night, a respected teacher of the law with a lifetime of religious achievement behind him. Jesus tells him that none of it counts as a starting point, and that what he needs is a beginning he cannot give himself. Born again is not church jargon for trying harder. It is Jesus saying that the change required is one you receive rather than one you produce.