When you get past the most basic "Hello world!" examples, your code will reach a level of complexity where you can't necessarily figure out what's going on just by running it. What you need, is some black magic, which allows you to open the virtual hood of your application while it's running and see what's going on. Debugging is that magical tool, and as soon as you learn the most basic steps of it, you will wonder how you ever lived without it. It's a tool that every good programmer should understand, simply because it's almost impossible to fix bugs in complex code without it.
The most basic type of debugging, which is still being used by even advanced programmers, is sometimes called "print debugging" - a simple procedure, where you make your application print a piece of text or a number somewhere, allowing you to see which part of your code has been reached and what your variables contain. With C#, you can use the Console.Write() method, to output the contents of a variable or a simple status message, which will be printed to the console. That can be enough for some situations, but if you're using a nice IDE like Visual Studio or one of the Express versions, you have much stronger tools to your disposal, and they are every bit as easy to use, once you learn the most basic principles. In the next couple of chapters, we will guide you through the debugging possibilities of your IDE and after that, you will be a much stronger programmer.