Variable testing is a process of testing different elements in an email message to see which ones produce the best results. For example, you might test different subject lines, images, or calls to action to see which ones get more people to open and click through the email. Variable testing can be used to improve the performance of email campaigns and make sure that messages are being seen and interacted with by the right people. By testing different variables, you can learn what works best for your audience and make sure that your email campaigns are effective.