Regression testing means it is the type of testing in which one will conduct testing on an already tested functionality again and again. Regression testing will be conducted in two situations.
1. If the test engineer find any defect on one functionality then after the rectification of that defect from the development department again testing on that defected functionality and retesting the related functionalities of that defected functionality.
2. If the new features are added to that application testing on that new feature functionality and also the related functionality of that new features to be tested.