The role of conversion rate optimization (CRO) in marketing is to improve the percentage of visitors that complete desired actions on a website, like making a purchase or signing up for a newsletter. CRO involves analyzing user behavior, conducting tests (like A/B tests), and making data-driven adjustments to enhance the user experience. By focusing on elements such as calls to action, page layout, and overall design, businesses can increase their conversion rates, leading to higher revenue and improved customer engagement.