Internet marketers are looking to achieve certain actions or results from visitors to a web page or site. The conversion rate is the fraction of those total visitors who go ahead and complete the desired action.
For example a shoe sales site runs a blog about healthy living and running in the hope that their quality content about fitness and the best running shoes will lead to readers visiting their shoe sales partner site and buying shoes. If a thousand visitors a month come to the site and ten go on to buy some trainers then the conversion rate of the blog is 1%.
As with all metrics it will be extremely important for successful marketers to test various different strategies and designs to optimise the conversion rate. A link to the specific types of trainers being discussed with a discount coupon showing might lead to a higher conversion rate for the blog than simply a link to the homepage of the shoe store section of the site with dozens of shoes highlighted. Whilst this might be intuitive and not require testing, other less obvious improvements would only be found through experience and testing.