A/B test - is a methology to improve existing ads/landing pages/graphics that provide better results. It is a method to provide consistance improvements(so it takes time) to results, conversion rates, sales or any quantitative results.
Prerequisite: basic convesion rates, numeric focused mind, ab-test glossary
For example, in the contact us form, someone suggested that large submit button will have higher form conversation. We do not know which is true until we tested it. We can apply A/B test here to verify the suggestion above.