Ok, you've written some code, and it seems to work. How can you be sure that it works? It's a busy, complicated, dangerous world out there, and software has to work in lots of different environments.
How can you gain confidence about your code? How can you make your code more reliable?
There are a lot of techniques available to developers today; I'll talk about several of them: Unit tests, static analysis, runtime analysis, fuzzing, decoding compiler warnings and probably others.