A/B testing is a way of trialing different strategies at the same time. To use A/B testing you can test headlines, advertisement copy or email subjects to see what your audience responds best to. Many companies constantly use A/B testing to see what they can improve on.
For example…
You may have two Facebook ads; one that says “Visit our website to view our specials” and another saying “Want to save, TODAY? Find out how!” If one of these performs significantly better than the other, you can drop the lower performing text. Keep up the experiment by then testing your more successful wording against another option, e.g. “Discover the secret to saving hundreds of dollars.”