Regression testing is a crucial step in ensuring that code changes do not introduce new bugs or break existing functionality, allowing developers to verify application behavior and ensure it works as expected before deployment. It can be performed manually or automated, with automated methods being more efficient but also requiring manual review to distinguish between intentional and unintentional changes. Regression testing is an essential part of software development, helping to reduce the number of bugs released and ensuring that new features and bug fixes do not compromise existing functionality.