CRO is the method of improving the performance of a website, not by directing more traffic to it, but instead by getting a higher proportion of the existing traffic to convert.
The underlying assumption is that each website has a purpose and is trying to get its visitors to take some action. Therefore it should have a Call To Action (CTA). CRO uses different methods to get people to follow the CTA.
Sometimes it means making the CTA more prominent, or more explicit. Sometimes, it means removing distractions or friction in the process.