A multivariate campaign helps you determine the best possible combination of content and also explains which content element most contributes to campaign success.
Instead of testing every possible combination, a multivariate (MVT) campaign requires that a subset of all possible combinations be tested. That subset determines the best possible combination even if that combination wasn't actually tested. In this way, less traffic is required than if you were to test all possible combinations.
A common use of multivariate tests is to optimize an entire page after you've used an A/B test to determine an optimal layout. With the multivariate test, you can optimize the individual assets on the page (such as the main image, the headline treatment, or main content promotion).