Good is the absence of Bad. It applies to everything in life and code is no exception. Thus,

Good code is one that doesn't have a smell! So, it is important for you to see and understand what a code smell is. No joke intended.

Think about it. You say you are happy only because you are NOT sad or have got ridden of your sadness. Likewise, you call something a good code, when you don't see anything bad in the code. Every quality aspect of code (maintainability, readability, etc) can be attained by getting rid of code smell. And believe it or not this doesn't happen the very first time - it happens iteratively :)