The practice of making more of your traffic convert into customers.
CRO is systematic experimentation on your site, funnel, and copy to lift conversion rates. It's research (what are users stuck on?) plus testing (A/B tests, multivariate tests) plus iteration. A CRO team might test headlines, button colors, form lengths, checkout flows, anything that might impact the conversion rate.
CRO is usually the cheapest path to more revenue. You already have traffic; you already have a product; improving conversion is pure upside. Established businesses should have CRO as a standing function, not a project.