To find out what conversion rate optimization is, Wikipedia gives the following definition:
In internet marketing, conversion optimization, or conversion rate optimization (CRO) is a system for increasing the percentage of visitors to a website that convert into customers, or more generally, take any desired action on a webpage. It is commonly referred to as CRO.
Using more familiar terms, conversion rate optimization represents the effort put on increasing the number of goals from the same traffic. A goal can be defined as a desired action, in online shopping being the same with purchase.