In the context of SEO, Conversion Rate Optimization (CRO) is a term given to the science of converting more of your visitors into actual customers. CRO is simply focusing on the 'conversions' as the core metric when optimizing any particular page.
Conversion Rate Optimization (CRO) consists of a series of actions and strategies designed to enable Internet merchants and marketers to easily and consistently gain remarkable increases in website conversions, and hence an increase in new customers, ongoing clients, and/or site trials.