Test-Driven Development, or TDD, is a disciplined approach to writing software that revolves around a simple but powerful cycle: Red-Green-Refactor. This process begins by writing a test that describes the desired behavior of a piece of code, even before the code itself exists. At this stage, the test will naturally fail, signaling the "Red" phase. Next, you write just enough code to make the test pass, moving into the "Green" phase. Finally, you improve and clean up the code without changing its behavior—this is the "Refactor" phase. By repeating this loop, you build up your application incrementally with confidence that each part works as intended.

The Red-Green-Refactor cycle encourages you to think about requirements and design before implementation. Since you always start by writing a test, you clarify what the code should do and why. This leads to better-designed code that is easier to maintain and extend. TDD also helps catch bugs early, as every new feature or change is immediately verified by tests. Over time, this reduces the number of defects and ensures that your codebase remains reliable, even as it grows.