A/B testing of subject lines involves creating two versions of the same email, with the only difference being the subject line. This allows marketers to compare the performance of two different subject lines and determine which one resonates more with their audience. The two versions of the email are sent to a randomly selected sample of the email list, with each version being sent to a roughly equal number of recipients. This helps to eliminate any biases and ensure that the results are representative of the target audience.