The concept is simple. Just divide your email list into 2 test groups - A and B. Create two versions of your email that are exactly the same except for one variable (this allows you to more accurately measure the impact of the element you’re testing). Then send one version of your email to group A and the other version to group B.
By A/B split testing various elements of your email marketing campaigns you’re able to optimise your campaigns for higher opens, clicks and conversions. Remember though, things change all the time in the email space so it’s important to test regularly.