An Ad Sequence typically refers to a YouTube video campaign or Facebook advertising campaigns where a series of ads are shown in a specific sequence at different times. Ad sequences allow a brand to tell a larger story with their ads because each video can build on the previous one. An ad that is shown to someone at the top of the funnel can (and most times, should) look significantly different than an ad shown to someone closer to the bottom of the funnel. Ad sequences allow you to lead people through the funnel one ad at a time.
