Everyone writes code that has bugs in it, but no one intends to write code that has bugs. The bugs occur because there's a disparity between what you think your code is doing and what your code is actually doing. The quote you give is an suggestion for how to reduce how often this occurs by considering why you should be confident about your intent being properly conveyed by the code you've written.

It might help to think of this almost as debugging your program as you write it rather than only after you run it. Debugging a program is essentially the same process of trying to figure out why the program isn't doing what you think of doing. While there's information you can get by inspecting a running program or its outputs, there's often some amount of consideration you need to do in order to figure out why you think certain behavior might be happening. There's no fundamental reason this consideration can't happen before you actually run the program; even as you're writing the code, you can think about all the ways it could potentially go wrong, and why you're confident that this isn't the case. If you can't come up with a strong argument for that, it's a good sign you might need to reconsider your approach!