It is understandable that you may not recognise what a "CV" is. If you're American (or learned American English), you will probably use the word "resumé" instead.
But "published code for review" seems fairly self explanatory: they want to see your public record (probably on Github) so that they can evaluate your abilities by looking at your actual code.
If you have a Github account, I suggest you provide them with your ID so that they can look you up and see what you've been doing. Likewise with Bitbucket or Gitlab or elsewhere. Also if you have contributed to any third-party open source projects you should give direct links to them as well.
Another source of public code that you could share could be blog posts where you've discussed coding techniques, or answers on StackOverflow where you've given a code solution to a problem. Either of these would also be a good way to showcase yourself to them.
If you don't have any code that you've published in pubic, then honestly, you may struggle to get the role; if they're asking to see your public code, then it implies that they place value in you having some public code or open source contributions, so you are already putting yourself at a disadvantage. You can (and should) still apply, and either provide them directly with some code samples that aren't public or provide a good explanation of why you can't do that. Make sure your CV/resume is as strong as you can make it, to compensate for the lack of code.