Regression testing is checking that old features of a program still work after you change something new. It makes sure fixes or updates don’t break what was already working.

How it works

When developers add new code or patch bugs, those changes might cause hidden problems elsewhere. Regression tests run the old test cases again to confirm that everything still behaves the same. If a test that once passed now fails, it means the update introduced a regression.

Analogy

It's like fixing a leaky pipe in your kitchen. After the repair, you don't just check that spot - you also turn on taps in the bathroom and garden to see if the rest of the plumbing still works.