Split Testing also referred to as A/B testing, is when you test more than one version of something to see which gains the best audience reaction.
This is commonly used in email marketing when testing subject lines or message previews, and also when running display ads by changing the images or text used in the ads.
Typically, the test is run for a short period of time (for example, hours for emails and days for display ads) and the winning variant is either what is used for the rest of the campaign, or what is used in the next split test.