Ad Experiments allow to you test various versions of ads and landing pages to find the best performing one. If you are testing variations of a single element like change in Headline 1, it is known as A/B test. if you are testing multiple variations of multiple elements, it is known as Multivariate test.
It is recommended to select CTR as the testing parameter while testing ad copies and Conv Rate as the parameter while testing landing pages.
Find the steps to create your first experiment here.