Split Testing or A/B Testing is a common method of optimizing marketing creative or conversion processes. In short, the idea is two run two different parallel campaigns or creatives at the same time with only a single variable different. This allows you to isolate that variable to determine what is most effective in driving your desired action.
In the case of email marketing, split testing can be used to determine the most effective subject line to drive higher open ratios or the best body text or images to increase click-thru rates.
You can even run a split test on a small portion of your list and send the winning email to the rest of your list automatically after a predefined period of time!
Unlike other email marketing software which limits you to A/B split testing, you can test unlimited variations in a single split test.