CRO is the method of using analytics and user feedback in order to improve the performance of a website.
Conversion rate optimisation (CRO) is a structured and systematic approach to improving the performance of a website based on informed insights, typically analytics and user feedback. It is defined by the websites objectives and KPIs.
Basically CRO is the ability to make people act once they arrive on the website. It typically involves elements of visual design, copywriting, user experience and so on.