A conversion occurs when a visitor performs a desired action on your website. For ecommerce this could be the completion of a sale, subscription to a newsletter, etc. Therefore, testing changes to your website and their effects on conversion rates is fundamental in the growth and development of your ecommerce store.
A great source of information on optimizing Zen Cart for higher conversion rates can be found at ZenCartOptimization.com. The author, Eric, of that website has also been kind enough to publicly release a great module called Simple Google Analytics which allows Zen Cart stores to track information, including conversions, on the Google Analytics website. Another great module written by Andrew Berezin called Google Analytics by Andrew is available for download through the official Zen Cart website.