Regeneration refers to the concept of spiritual rebirth or being born again. It's the act of God whereby an individual's heart and nature are transformed, leading to a new spiritual life in Christ. 

In Calvinism, regeneration is often associated with the idea that because of the original sin, humans are unable to respond positively to God, the Gospel, or even a direct call from Jesus on their own. They require God's sovereign intervention to bring about a change in their spiritual condition through regeneration before they are then, and only then, able to have faith.