CRO is Conversion Rate Optimization.
With CRO we use data, testing, and creativity to maximize the percentage of visitors who convert on a website. These conversions can be purchases, newsletter signups, quote requests, phone calls, or any other action you would like your visitors to take. Optimizing your website to increase the conversion rate (the percentage of visitors who take the desired action) ensures your website is not wasting any of the valuable traffic you are sending to it.
To get a high converting site, you may need to improve numerous aspects including design, content, and overall user experience. Before making these improvements, you must determine what obstacles are causing your visitors to leave and remove those obstacles. Many marketers remove these obstacles by making opinion-based decisions relying on best practices. This is not CRO. Properly optimizing your site for conversions means relying on real data and rigorous testing to make informed improvements to your website.