A regression test checks that something which used to work still works after a change. It is like checking that the lights still turn on after an electrician repairs a nearby socket.
If a developer changes the sign-in page, a regression test might check that password reset still works too. A small edit can sometimes affect another feature that shares the same code.
Teams often repeat these checks automatically. A passing regression test gives useful confidence, but it only covers the situations the test actually checks.