Code quality refers to how well written the code is. Quality code is clear, simple, well tested, bug-free, refactored, documented, and performant. But the primary measure of high-quality code is its compliance with the specifications provided by the individual/organization who needs the code.
Code quality is like writing quality.
You can write a paper in 7th grade, working on your very first essay. You can also write a paper at 50 years old being a professional author.
Both papers may be working to explain the same concept or tell the same story, but you can likely distinguish that the quality of the writing of the author is better than the 7th grader.
Code quality is a reflection of the proficiency of the developer and a signal as to the quality of the end product.
Before getting started on your product, you should know what level of quality you are looking to build with. If you're going for an MVP, then you probably don't need high quality, but if you already have a waitlist of over 1k individuals, then you probably want to have higher code quality.
Once your product is live, it is important to maintain and improve your code quality. Sometimes you need to refactor your codebase to make it run more smoothly. Other times, if your code is really poor quality, you may be better off just starting over from scratch.