What does regression test mean?

Regression testing basically perform after completing of retesting. The main purpose of regression testing is to check the impact of modification. Whether still our application is acting stable.

Its necessary to perform a regression testing because sometimes it happened after retesting or while fixing bug developer fixed the bug and missed out something on other code or on dependent code

Regression testing means testing your software/website repeatedly. The main reason for it is to make sure there aren't any new bugs introduced.

Typically, regression tests will be automated, to reduce the cost of rerunning the test. The more high value test cases you can construct, the better. This is one example of a Play and Record regression testing platform

I like this definition of regression testing:

[regression testing] tells you if a previously written and tested code broke after you’ve added an update or a fix

[...] it helps you notice if you’ve unknowingly introduced bugs to your software while adding new code. New bugs of this kind are called regressions.

Basically, a regression is returning to a state where your application has bugs.

Regression testing is an activity performed to ensure the different functionalities of the system are still working as expected and the new functionalities added did not break any of the existing ones.

Secondly, you generally write automated tests or do manual testing to the above mentioned testing. It could be a combination of Unit/API/UI tests that are run on a daily basis. Regression testing can be performed in various phases of the SDLC, it all depends on the context.

Hopefully this gives an idea on what is regression testing.