Explanation in layman terms

Imagine patching a leaky pipe by wrapping tape around it. If you never pressure-test the fix, the moment water flows hard again, the leak reappears. A test is that pressure test for software: it forces the code to prove the bug is gone--every build, forever. Writing the test right after (or before) the fix means you lock the door behind the escaping bug so it can't re-enter the house.

Gain Deeper Understanding

Action	What you'll learn
Read Martin Fowler's Test Pyramid article	The strategic approach to testing that balances different types of tests and why unit tests form the foundation for catching bugs quickly
Watch the video "Why you should write tests first" by Uncle Bob	The psychological and practical reasons why writing tests after the fact often leads to untested code and hidden bugs
Read the article "Test-Driven Bugfixing"	A practical approach for applying TDD principles when fixing bugs

Put it into practice?