A/B testing, in the context of email, is the idea of sending one variation of your campaign to a subset of your subscribers, and a different variation to another subset of subscribers, with the ultimate goal of working out which variation of the campaign garners the best results.
A/B testing can vary in complexity and simple A/B Tests can include sending multiple subject lines to test which one generates more opens, while more advanced A/B testing could include testing completely different email templates against each other to see which one generates more click-throughs.
If you’re using email service providers like Campaign Monitor or Mailchimp, A/B testing your campaigns is really easy. You can use the email builder to create 2 different variations of your email, and your ESP will automatically send it to 2 different subsets of your list to see which variation performs best. Once the test has concluded and the winning version has been found, it’ll automatically send the winning version to the rest of your list.