Can you explain what regression testing means and what its essence is?

Regression testing is a type of testing conducted to verify that changes in the code (such as bug fixes, new features, or improvements) have not introduced new errors in the already functioning features. Its essence is to ensure that previously fixed and tested parts of the program continue to work correctly after modifications.

Typically, regression testing involves re-running a set of tests that cover the main functions of the application. This helps identify side effects and prevent new defects.