Lets me guess, do you mean what is CRO in Web Marketing?
"In internet marketing, conversion optimization, or conversion rate optimization (CRO) is the method of creating an experience for a website or landing page visitor with the goal of increasing the percentage of visitors that convert into customers. It is also commonly referred to as CRO."
http://en.wikipedia.org/wiki/Conversion_optimization
Pease next time give me some more details