Well so before you code it you still have an idea of what you want the program to do, right? So you write a test for the program to pass or fail based on that idea of functionality, and then you write the program to pass the test.

So for something simple like programming a calculator you might write test code that verifies whether your addition function properly adds two numbers together then write the actual addition function.

Later on as you continue to build the program your addition test will still be out there verifying that you haven't broken anything with subsequent code.

Some people will tell you that TDD tends to work better with established codebases in corporate environments where you have huge interrelated programs and maybe hundreds or even thousands of developers working concurrently as opposed to simple projects or startups where you might want to prioritize having a product set out before you start to implement rigorous testing requirements.

A lot of people don't like TDD because they see it as extra overhead and don't want to spend time writing test code when they could be writing "real code."

Proponents of TDD tend to point to the fact that it contributes to stability in the overall codebase and allows you to quickly and easily find and diagnose problems, and it can make you a better developer to think ahead rather than just dumping code into the codebase and assuming it's going to work.