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.
The CTA is the primary usier interface element (button, link, form...) that asks the user to take the action the website would like its visitors to take. This results in a "Conversion".
Examples: "Buy now" button, "Sign up", "Download now", "Subscribe here"...
The bounce rate is the percentage of visitors who leave a web site after viewing a single page.
This is widely regarded as a bad thing because it may mean that visitors did not find what they were looking for and "bounced" away to another site. They left almost immediately.
However, this is a broken metric in many cases. For example if you were looking for opening times of a retailer in your city and you found that info right on the homepage of the retailer, you will technically leave the site after a single page view. However it does not mean you did not find what you were looking for.
Furthermore you may have visited some competing sites in a row, "bouncing" from on e to the next, still finding the exact information (closing times) on each of them in a single page view each.