"Success event" is an Adobe Analytics-specific terminology that basically means "conversion".
For example, let's say you want your website users to submit a form. Then when the form has been submitted successfully, you track a counter "success event" to Adobe Analytics. That causes Adobe Analytics to increment the count of the "success event". You will then be able to report on the number of times that the "success event" was tracked, which implies that you can report on the number of form submissions.